[FIXED] How can I get two lines to plot as the same colour within a loop?

Issue

I want to produce a graph showing the error of a numerical function for various convergence methods/values on the same graph.

I am using a while loop to increment through different values of convergence condition and plot the error on a graph, once for each convergence method (labelled with the suffix A & B).

The problem is that each time I call plot in matplotlib the line displays as a different colour. I want the same colour for each convergence value (ie to plot the lines as the same colour if they share a convergence value) to make it easier for the reader to see which lines are connected.

An example of what my code currently produces.

Whereas I would like the coloured and black dotted line in each pair to be the same colour. I haven't included the definitions of the functions, but the code I'm using to produce the graph is

conv = 1e-0
h=0.1

while conv>1e-4:

    errorlist_A, errorlist_B = [],[]
    gridsize = []
    grid_x = 40


    while grid_x <= 100:

        analytical = (np.ones((grid_x,grid_x)))*edge_val
        P = np.zeros((grid_x,grid_x))

        gridsize.append(grid_x)


        solA = solvelap_avg_rand(grid_x,grid_x,edge_val,P,h,conv)
        solB = solvelap_max_rand(grid_x,grid_x,edge_val,P,h,conv)

        errorlist_A.append(errorfunc(grid_x,grid_y,analytical,solA,edge_val))
        errorlist_B.append(errorfunc(grid_x,grid_y,analytical,solB,edge_val))

        grid_x +=5
        print(grid_x)

    plt.plot(gridsize,errorlist_A, label = "{0:.1g}".format(conv))
    plt.plot(gridsize,errorlist_B, label = "{0:.1g}".format(conv) , linestyle = "--", color = "k")

    conv *=0.1


plt.title("Fig:6 Error per Node Vs. Grid Width")
plt.xlabel("Grid Width")
plt.ylabel("Average Error Per Node (Log10)")
plt.yscale("log")
plt.legend()
plt.show() 

Note: I'm aware you can set the colours of the lines manually as an argument, but I'm not sure how to do this in a loop.

In addition, if there's anyway to prevent the legend from blocking the plots that would also be very helpful.

Solution

I would use a colormap to define automatically the color. However, since you don't know in advance how many curves there will be, we can plot all the curves and color them afterwards.

Modify the following lines in your code:

# Remove the `color=` parameter here, it will be overwritten anyway.
plt.plot(gridsize, errorlist_A, label="{0:.1g}".format(conv))
plt.plot(gridsize, errorlist_B, label="{0:.1g}".format(conv), linestyle = "--")

Then add those right before plt.show():

# After you plotted them, apply a random color to each curve.
import matplotlib.cm as cm
colormap = cm.get_cmap("plasma") # The various colormaps can be found here: https://matplotlib.org/3.1.0/tutorials/colors/colormaps.html
ax = plt.gca()
lines = ax.lines
N = len(lines)
for n in range(0, N, 2): # For each two-lines made via `plt.plot(...)`:
    random_color = colormap(n/N) # This function takes a number between 0 and 1 and returns a color.
    lines[n].set_color(random_color)
    lines[n+1].set_color(random_color)

Answered By - Guimoute