Shallow Thoughts : tags : matplotlib

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Thu, 30 May 2019

Plotting a Sequence of Graphs in Matplotlib 3D

A friend and I were talking about temperature curves: specifically, the way the temperature sinks in the evening but then frequently rises again before it really starts cooling off.

I thought it would be fun to plot the curve of temperature as a function of time over successive days, as a 3-D plot. I knew matplotlib had a way to do 3D plots, but I've never actually generated one.

Well, it turns out there are lots of examples, but they all start by generating mysterious data blobs, and none of them explain the structure of the data they're using, and the documentation has mysterious parameters like "zs" that aren't explained anywhere. So getting something that worked was a fiddly process. Creating a color version, to distinguish the graphs better, was even more fiddly.

[Plotting a series of graphs using matplotlib 3d] So I wrote an example that I hope will make it a little clearer for anyone trying to use this library. It can plot using just lines:

[Plotting a series of graphs using matplotlib 3d, color option] ... or it can plot in color, cycling colors manually because by default matplotlib makes adjacent colors similar, exactly the opposite of what you'd want:

Here's the demo: on GitHub.

... Except there's a Bug

All is not perfect. Axes3D gets a bit confused sometimes about which layer is supposed to be in front of which other layer. You can see that on the two plots: in both cases, the fourth and fifth layers from the front are reversed, so the fifth layer is drawn in front of the fourth layer. I haven't yet found anyone in the matplotlib organization who seems to know much about Axes3D; eventually I'll file a bug but I want to write a shorter, clearer test case to illustrate the problem. Still, even with the bugs it's a useful technique to know.

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[ 09:57 May 30, 2019    More programming | permalink to this entry | comments ]

Thu, 19 Jan 2017

Plotting Shapes with Python Basemap wwithout Shapefiles

In my article on Plotting election (and other county-level) data with Python Basemap, I used ESRI shapefiles for both states and counties.

But one of the election data files I found, OpenDataSoft's USA 2016 Presidential Election by county had embedded county shapes, available either as CSV or as GeoJSON. (I used the CSV version, but inside the CSV the geo data are encoded as JSON so you'll need JSON decoding either way. But that's no problem.)

Just about all the documentation I found on coloring shapes in Basemap assumed that the shapes were defined as ESRI shapefiles. How do you draw shapes if you have latitude/longitude data in a more open format?

As it turns out, it's quite easy, but it took a fair amount of poking around inside Basemap to figure out how it worked.

In the loop over counties in the US in the previous article, the end goal was to create a matplotlib Polygon and use that to add a Basemap patch. But matplotlib's Polygon wants map coordinates, not latitude/longitude.

If m is your basemap (i.e. you created the map with m = Basemap( ... ), you can translate coordinates like this:

    (mapx, mapy) = m(longitude, latitude)

So once you have a region as a list of (longitude, latitude) coordinate pairs, you can create a colored, shaped patch like this:

    for coord_pair in region:
        coord_pair[0], coord_pair[1] = m(coord_pair[0], coord_pair[1])
    poly = Polygon(region, facecolor=color, edgecolor=color)

Working with the OpenDataSoft data file was actually a little harder than that, because the list of coordinates was JSON-encoded inside the CSV file, so I had to decode it with json.loads(county["Geo Shape"]). Once decoded, it had some counties as a Polygonlist of lists (allowing for discontiguous outlines), and others as a MultiPolygonlist of list of lists (I'm not sure why, since the Polygon format already allows for discontiguous boundaries)

[Blue-red-purple 2016 election map]

And a few counties were missing, so there were blanks on the map, which show up as white patches in this screenshot. The counties missing data either have inconsistent formatting in their coordinate lists, or they have only one coordinate pair, and they include Washington, Virginia; Roane, Tennessee; Schley, Georgia; Terrell, Georgia; Marshall, Alabama; Williamsburg, Virginia; and Pike Georgia; plus Oglala Lakota (which is clearly meant to be Oglala, South Dakota), and all of Alaska.

One thing about crunching data files from the internet is that there are always a few special cases you have to code around. And I could have gotten those coordinates from the census shapefiles; but as long as I needed the census shapefile anyway, why use the CSV shapes at all? In this particular case, it makes more sense to use the shapefiles from the Census.

Still, I'm glad to have learned how to use arbitrary coordinates as shapes, freeing me from the proprietary and annoying ESRI shapefile format.

The code: Blue-red map using CSV with embedded county shapes

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[ 09:36 Jan 19, 2017    More programming | permalink to this entry | comments ]

Sat, 14 Jan 2017

Plotting election (and other county-level) data with Python Basemap

After my arduous search for open 2016 election data by county, as a first test I wanted one of those red-blue-purple charts of how Democratic or Republican each county's vote was.

I used the Basemap package for plotting. It used to be part of matplotlib, but it's been split off into its own toolkit, grouped under mpl_toolkits: on Debian, it's available as python-mpltoolkits.basemap, or you can find Basemap on GitHub.

It's easiest to start with the example that shows how to draw a US map with different states colored differently. You'll need the three shapefiles (because of ESRI's silly shapefile format): st99_d00.dbf, st99_d00.shp and st99_d00.shx, available in the same examples directory.

Of course, to plot counties, you need county shapefiles as well. The US Census has county shapefiles at several different resolutions (I used the 500k version). Then you can plot state and counties outlines like this:

from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt

def draw_us_map():
    # Set the lower left and upper right limits of the bounding box:
    lllon = -119
    urlon = -64
    lllat = 22.0
    urlat = 50.5
    # and calculate a centerpoint, needed for the projection:
    centerlon = float(lllon + urlon) / 2.0
    centerlat = float(lllat + urlat) / 2.0

    m = Basemap(resolution='i',  # crude, low, intermediate, high, full
                llcrnrlon = lllon, urcrnrlon = urlon,
                lon_0 = centerlon,
                llcrnrlat = lllat, urcrnrlat = urlat,
                lat_0 = centerlat,

    # Read state boundaries.
    shp_info = m.readshapefile('st99_d00', 'states',
                               drawbounds=True, color='lightgrey')

    # Read county boundaries
    shp_info = m.readshapefile('cb_2015_us_county_500k',

if __name__ == "__main__":
    plt.title('US Counties')
    # Get rid of some of the extraneous whitespace matplotlib loves to use.
    plt.tight_layout(pad=0, w_pad=0, h_pad=0)
[Simple map of US county borders]

Accessing the state and county data after reading shapefiles

Great. Now that we've plotted all the states and counties, how do we get a list of them, so that when I read out "Santa Clara, CA" from the data I'm trying to plot, I know which map object to color?

After calling readshapefile('st99_d00', 'states'), m has two new members, both lists: m.states and m.states_info.

m.states_info[] is a list of dicts mirroring what was in the shapefile. For the Census state list, the useful keys are NAME, AREA, and PERIMETER. There's also STATE, which is an integer (not restricted to 1 through 50) but I'll get to that.

If you want the shape for, say, California, iterate through m.states_info[] looking for the one where m.states_info[i]["NAME"] == "California". Note i; the shape coordinates will be in m.states[i]n (in basemap map coordinates, not latitude/longitude).

Correlating states and counties in Census shapefiles

County data is similar, with county names in m.counties_info[i]["NAME"]. Remember that STATE integer? Each county has a STATEFP, m.counties_info[i]["STATEFP"] that matches some state's m.states_info[i]["STATE"].

But doing that search every time would be slow. So right after calling readshapefile for the states, I make a table of states. Empirically, STATE in the state list goes up to 72. Why 72? Shrug.

    states = [None] * MAXSTATEFP
    for state in m.states_info:
        statefp = int(state["STATE"])
        # Many states have multiple entries in m.states (because of islands).
        # Only add it once.
        if not states[statefp]:
            states[statefp] = state["NAME"]

That'll make it easy to look up a county's state name quickly when we're looping through all the counties.

Calculating colors for each county

Time to figure out the colors from the Deleetdk election results CSV file. Reading lines from the CSV file into a dictionary is superficially easy enough:

    fp = open("tidy_data.csv")
    reader = csv.DictReader(fp)

    # Make a dictionary of all "county, state" and their colors.
    county_colors = {}
    for county in reader:
        # What color is this county?
        pop = float(county["votes"])
        blue = float(county["results.clintonh"])/pop
        red = float(county["Total.Population"])/pop
        county_colors["%s, %s" % (county["name"], county["State"])] \
            = (red, 0, blue)

But in practice, that wasn't good enough, because the county names in the Deleetdk names didn't always match the official Census county names.

Fuzzy matches

For instance, the CSV file had no results for Alaska or Puerto Rico, so I had to skip those. Non-ASCII characters were a problem: "Doña Ana" county in the census data was "Dona Ana" in the CSV. I had to strip off " County", " Borough" and similar terms: "St Louis" in the census data was "St. Louis County" in the CSV. Some names were capitalized differently, like PLYMOUTH vs. Plymouth, or Lac Qui Parle vs. Lac qui Parle. And some names were just different, like "Jeff Davis" vs. "Jefferson Davis".

To get around that I used SequenceMatcher to look for fuzzy matches when I couldn't find an exact match:

def fuzzy_find(s, slist):
    '''Try to find a fuzzy match for s in slist.
    best_ratio = -1
    best_match = None

    ls = s.lower()
    for ss in slist:
        r = SequenceMatcher(None, ls, ss.lower()).ratio()
        if r > best_ratio:
            best_ratio = r
            best_match = ss
    if best_ratio > .75:
        return best_match
    return None

Correlate the county names from the two datasets

It's finally time to loop through the counties in the map to color and plot them.

Remember STATE vs. STATEFP? It turns out there are a few counties in the census county shapefile with a STATEFP that doesn't match any STATE in the state shapefile. Mostly they're in the Virgin Islands and I don't have election data for them anyway, so I skipped them for now. I also skipped Puerto Rico and Alaska (no results in the election data) and counties that had no corresponding state: I'll omit that code here, but you can see it in the final script, linked at the end.

    for i, county in enumerate(m.counties_info):
        countyname = county["NAME"]
            statename = states[int(county["STATEFP"])]
        except IndexError:
            print countyname, "has out-of-index statefp of", county["STATEFP"]

        countystate = "%s, %s" % (countyname, statename)
            ccolor = county_colors[countystate]
        except KeyError:
            # No exact match; try for a fuzzy match
            fuzzyname = fuzzy_find(countystate, county_colors.keys())
            if fuzzyname:
                ccolor = county_colors[fuzzyname]
                county_colors[countystate] = ccolor
                print "No match for", countystate

        countyseg = m.counties[i]
        poly = Polygon(countyseg, facecolor=ccolor)  # edgecolor="white"

Moving Hawaii

Finally, although the CSV didn't have results for Alaska, it did have Hawaii. To display it, you can move it when creating the patches:

    countyseg = m.counties[i]
    if statename == 'Hawaii':
        countyseg = list(map(lambda (x,y): (x + 5750000, y-1400000), countyseg))
    poly = Polygon(countyseg, facecolor=countycolor)
The offsets are in map coordinates and are empirical; I fiddled with them until Hawaii showed up at a reasonable place. [Blue-red-purple 2016 election map]

Well, that was a much longer article than I intended. Turns out it takes a fair amount of code to correlate several datasets and turn them into a map. But a lot of the work will be applicable to other datasets.

Full script on GitHub: Blue-red map using Census county shapefile

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[ 15:10 Jan 14, 2017    More programming | permalink to this entry | comments ]

Sun, 11 May 2014

Sonograms in Python

I went to a terrific workshop last week on identifying bird songs. We listened to recordings of songs from some of the trickier local species, and discussed the differences and how to remember them. I'm not a serious birder -- I don't do lists or Big Days or anything like that, and I dislike getting up at 6am just because the birds do -- but I do try to identify birds (as well as mammals, reptiles, rocks, geographic features, and pretty much anything else I see while hiking or just sitting in the yard) and I've always had trouble remembering their songs.

[Sonogram of ruby-crowned kinglet] One of the tools birders use to study bird songs is the sonogram. It's a plot of frequency (on the vertical axis) and intensity (represented by color, red being louder) versus time. Looking at a sonogram you can identify not just how fast a bird trills and whether it calls in groups of three or five, but whether it's buzzy/rattly (a vertical line, lots of frequencies at once) or a purer whistle, and whether each note is ascending or descending.

The class last week included sonograms for the species we studied. But what about other species? The class didn't cover even all the local species I'd like to be able to recognize. I have several collections of bird calls on CD (which I bought to use in combination with my "tweet" script -- yes, the name messes up google searches, but my tweet predates Twitter -- a tweet Python script and tweet in HTML for Android). It would be great to be able to make sonograms from some of those recordings too.

But a search for Linux sonogram turned up nothing useful. Audacity has a histogram visualization mode with lots of options, but none of them seem to result in a usable sonogram, and most discussions I found on the net agreed that it couldn't do it. There's another sound editor program called snd which can do sonograms, but it's fiddly to use and none of the many color schemes produce a sonogram that I found very readable.

Okay, what about python scripts? Surely that's been done?

I had better luck there. Matplotlib's pylab package has a specgram() call that does more or less what I wanted, and here's an example of how to use pylab.specgram(). (That post also has another example using a library called timeside, but timeside's PyPI package doesn't have any dependency information, and after playing the old RPM-chase game installing another dependency, trying it, then installing the next dependency, I gave up.)

The only problem with pylab.specgram() was that it shows the full range of the sound, both in time and frequency. The recordings I was examining can last a minute or more and go up to 20,000 Hz -- and when pylab tries to fit that all on the screen, you end up with a plot where the details are too small to show you anything useful.

You'd think there would be a way for pylab.specgram() to show only part of the spectrum, but that doesn't seem to be. I finally found a Stack Overflow discussion where "edited" gives an excellent rewritten version of pylab.specgram which allows setting minimum and maximum frequency cutoffs. Worked great!

Then I did some fiddling to allow for analyzing only part of the recording -- Python's wave package has no way to read in just the first six seconds of a .wav file, so I had to read in the whole file, read the data into a numpy array, then take a slice representing the seconds of the recording I actually wanted.

But now I can plot nice sonograms of any bird song I want to see, print them out or stick them on my Android device so I can carry them with me.

Update: Oops! I forgot to include a link to the script. Here it is: Sonograms in Python.

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[ 09:17 May 11, 2014    More programming | permalink to this entry | comments ]