区分python图中的一些点

Question

I have a plot in Python of the following

a = (1,2,3,4,5)
b = (1,2,3,4,5)
plot(a,b)

I want to differentiate some of the x axis points in the plot with a dots of unique colors for examples this points

c = (2,4)

I tried the following:

a = (1,2,3,4,5)
b = (1,2,3,4,5)
plot(a,b)
matplotlib.pyplot.scatter(a, c)

But I got the error "x and y must be the same size"

Answer 1

If you google "matplotlib scatter" or something like that, one of the first links will be this one , which says that the function gives "a scatter plot of y vs. x". So, the error message you shared makes sense, since the length of a is greater than the length of c . I hope it makes sense to you why what your original code gives that error.

Your problem is specific enough that there isn't an out-of-the-box solution in matplotlib that I'm aware of, so this will require the use of your own custom functions. I'll give one approach that might be helpful to you. I'm structuring my answer here so you can see how to solve the issue to your own specifications, instead of relying too heavily on copying & pasting other people's code, since the latter makes it harder for you to do exactly what you want to do.

To restate the problem in more concise terms: How can a matplotlib user plot a line, and then put markers on a subset of the line's points, specified by their x-values?

To begin, here is what your program might look like currently:

import matplotlib.pyplot as plt

x_coords = [ ] # fill in x_coords here
y_coords = [ ] # fill in y_coords here
marker_x_coords = [ ] # fill in with x coordinates of points you want to have markers

plt.plot(x_coords, y_coords) # plots the line

#### TODO: plot the markers ####

Now, you have the x-values of the points you want to put markers on. How might you get their corresponding y-values? Well, you can make a function that searches for the index of the x-value in x_coords , and then gives the corresponding value at the same index of y_coords :

def getYVals(x_coords_lst, y_coords_lst, marker_x_coords_lst):
    marker_y_coords = [] # start with an empty list
    for x_point in marker_x_coords_lst:
        point_index = x_coords_lst.index(x_point) # get the index of a point in the x list
        marker_y_coords.append(y_coords_lst[point_index]) # add the value of the y list at that index to the list that will be returned
    return marker_y_coords

This isn't the fastest method, but it is the clearest on what is happening. Here's an alternative that would give the same results but would likely perform faster computationally (it uses something called "list comprehension"):

def getYVals(x_coords_lst, y_coords_lst, marker_x_coords_lst):
    return [y_coords_lst[x_coords_lst.index(x_val)] for x_val in marker_x_coords_lst]

The output of either of the getYVals function above will work as the y values of the markers. This can be put in the y argument of plt.scatter , and you already have the x values of it, so from there you should be good to go!