Setting a fixed size for points in legend
legend, python, scatter-plot
Solution
I had a look into the source code of `matplotlib`. Bad news is that there does not seem to be any simple way of setting equal sizes of points in the legend. It is especially difficult with scatter plots (wrong: see the update below). There are essentially two alternatives:
- Change the `maplotlib` code
- Add a transform into the `PathCollection` objects representing the dots in the image. The transform (scaling) has to take the original size into account.
Neither of these is very much fun, though #1 seems to be easier. The `scatter` plots are especially challenging in this respect.
However, I have a hack which does probably what you want:
import matplotlib.pyplot as plt
import numpy as np
def rand_data():
return np.random.uniform(low=0., high=1., size=(100,))
# Generate data.
x1, y1 = [rand_data() for i in range(2)]
x2, y2 = [rand_data() for i in range(2)]
plt.figure()
plt.plot(x1, y1, 'o', label='first', markersize=np.sqrt(20.), c='b')
plt.plot(x2, y2, 'o', label='second', markersize=np.sqrt(35.), c='r')
# Plot legend.
lgnd = plt.legend(loc="lower left", numpoints=1, fontsize=10)
#change the marker size manually for both lines
lgnd.legendHandles[0]._legmarker.set_markersize(6)
lgnd.legendHandles[1]._legmarker.set_markersize(6)
plt.show()
This gives:
Which seems to be what you wanted.
The changes:
- `scatter` changed into a `plot`, which changes the marker scaling (hence the `sqrt`) and makes it impossible to use changing marker size (if that was intended)
- the marker size changed manually to be 6 points for both markers in the legend
As you can see, this utilizes hidden underscore properties (`_legmarker`) and is bug-ugly. It may break down at any update in `matplotlib`.
Update
Haa, I found it. A better hack:
import matplotlib.pyplot as plt
import numpy as np
def rand_data():
return np.random.uniform(low=0., high=1., size=(100,))
# Generate data.
x1, y1 = [rand_data() for i in range(2)]
x2, y2 = [rand_data() for i in range(2)]
plt.figure()
plt.scatter(x1, y1, marker='o', label='first', s=20., c='b')
plt.scatter(x2, y2, marker='o', label='second', s=35., c='r')
# Plot legend.
lgnd = plt.legend(loc="lower left", scatterpoints=1, fontsize=10)
lgnd.legendHandles[0]._sizes = [30]
lgnd.legendHandles[1]._sizes = [30]
plt.show()
Now the `_sizes` (another underscore property) does the trick. No need to touch the source, even though this is quite a hack. But now you can use everything `scatter` offers.
Problem
I'm making some scatter plots and I want to set the size of the points in the legend to a fixed, equal value. Right now I have this: ``` import matplotlib.pyplot as plt import numpy as np def rand_data(): return np.random.uniform(low=0., high=1., size=(100,)) # Generate data. x1, y1 = [rand_data() for i in range(2)] x2, y2 = [rand_data() for i in range(2)] plt.figure() plt.scatter(x1, y1, marker='o', label='first', s=20., c='b') plt.scatter(x2, y2, marker='o', label='second', s=35., c='r') # Plot legend. plt.legend(loc="lower left", markerscale=2., scatterpoints=1, fontsize=10) plt.show() ``` which produces this: The sizes of the points in the legend are scaled but not the same. How can I fix the sizes of the points in the legend to an equal value without affecting the sizes in the `scatter` plot?