MatPlotLib Scatter Plot Points All Have Same Color

Question

I'm creating a scatter plot in MatPlotLib and I'm trying to make the color of the points dependent on a third parameter (independent of X and Y). However, setting c=third_variable makes all of the dots the same color.

My plot data is in a dictionary with a tuple of x and y data as the key and the third parameter (frequency, ranges from 1 to about 1000) as its value. Sample item: {[2 10]: 50}

I want the colors of the points tied to frequency. As a starting place, I would like to make it such that dots with high frequency are darker and dots with low frequency are lighter.

Here's my current output: And here's my code:

for key in pairs_hash:
    plt.scatter(key[0], key[1], c=pairs_hash[key], cmap=plt.cm.coolwarm)

plt.show()

Thanks for the help!

More code is needed to have a closer look

user1767754
– user1767754

2017-06-26 22:18:32 +00:00
Commented Jun 26, 2017 at 22:18 — user1767754
– user1767754, Commented Jun 26, 2017 at 22:18

DYZ · Accepted Answer · 2017-06-26 23:21:33Z

2

I guess your major bad design choice is plotting the points in the loop one point at a time. Why not plot them all at once?

xy, z = zip(*pairs_hash.items())
x, y = zip(*xy)
plt.scatter(x, y, c=z, cmap=plt.cm.coolwarm)
plt.show()

edited Jun 26, 2017 at 23:21

answered Jun 26, 2017 at 22:23

DYZ

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3 Comments

Paul Murray Over a year ago

I made the changes you suggested but the results are the same. Could the issue be that MatPlotLib is interpreting c as being a sequence of floats (like you would give it if you had an array of number values) rather than a singular value and trying to apply that sequence to all the dots?

Paul Murray Over a year ago

Would you mind explaining the syntax of the first two lines a bit? I'm not a python programmer (school was C++, work is JavaScript) but I would guess just by looking that it's a question of tuples?

DYZ Over a year ago

x,y,...=zip(*X) takes a list of tuples X and creates a tuple of lists of the first elements, second elements, etc. It is the opposite of X=zip(x,y,...).

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