matplotlib path linewidth connected to figure zoom

Question

Is it possible to tie the linewidth of a matplotlib path to the figure zoom/scale level?

I am drawing a map where the matplotlib path (with bezier curves) draws the

Answer 1

To the best of my knowledge, there's no way to do this in matplotlib, as the stroke width of a line cannot be directly tied to data coordinates. (As you mentioned, you could connect a callback to the draw event and accomplish this. It would incur a large performance penalty, though.)

However, a quick workaround would be to use shapely to generate polygons by buffering your street paths.

As a quick example:

import shapely.geometry
import descartes
import matplotlib.pyplot as plt

lines = ([(0, 0), (1, 0), (0, 1)],
         [(0, 0), (1, 1)],
         [(0.5, 0.5), (1, 0.5)],
         )
lines = shapely.geometry.MultiLineString(lines)
# "0.05" is the _radius_ in data coords, so the width will be 0.1 units.
poly = lines.buffer(0.05)

fig, ax = plt.subplots()
patch = descartes.PolygonPatch(poly, fc='gray', ec='black')
ax.add_artist(patch)

# Rescale things to leave a bit of room around the edges...
ax.margins(0.1)

plt.show()

If you did want to take the callback route, you might do something like this:

import matplotlib.pyplot as plt

def main():
    lines = ([(0, 0), (1, 0), (0, 1)],
             [(0, 0), (1, 1)],
             [(0.5, 0.5), (1, 0.5)],
             )

    fig, ax = plt.subplots()
    artists = []
    for verts in lines:
        x, y = zip(*verts)
        line, = ax.plot(x, y)
        artists.append(line)

    scalar = StrokeScalar(artists, 0.1)
    ax.callbacks.connect('xlim_changed', scalar)
    ax.callbacks.connect('ylim_changed', scalar)

    # Rescale things to leave a bit of room around the edges...
    ax.margins(0.05)

    plt.show()

class StrokeScalar(object):
    def __init__(self, artists, width):
        self.width = width
        self.artists = artists
        # Assume there's only one axes and one figure, for the moment...
        self.ax = artists[0].axes
        self.fig = self.ax.figure

    def __call__(self, event):
        """Intended to be connected to a draw event callback."""
        for artist in self.artists:
            artist.set_linewidth(self.stroke_width)

    @property
    def stroke_width(self):
        positions = [[0, 0], [self.width, self.width]]
        to_inches = self.fig.dpi_scale_trans.inverted().transform
        pixels = self.ax.transData.transform(positions)
        points = to_inches(pixels) * 72
        return points.ptp(axis=0).mean() # Not quite correct...

main()