I am processing some data collected in a driving simulator, and I needed to plot the velocity against the location. I managed to convert the velocity and location values into 2 numpy arrays. Due to the settings of the simulator, the location array is continuously decreasing. The sample array is [5712.114 5711.662 5711.209 ... 3185.806 3185.525 3185.243]. Similarly, the velocity array is also decreasing because we were testing the brake behavior. Example array: [27.134 27.134 27.134 ... 16.87 16.872 16.874].

So, when I plot these 2 arrays, what I should see should be a negatively sloped line, and both x and y axis should have decreasing numbers. I used the code below to plot them:

plotting_x = np.array(df["SubjectX"].iloc[start_index-2999:end_index+3000])
plotting_y = np.array(df["Velocity"].iloc[start_index-2999:end_index+3000])
plt.plot(plotting_x, plotting_y, "r")

What I saw is a graph attached here. Anyone know what went wrong? Does Matplotlib not allow decreasing series? Thanks! Matplotlib plot

>Solution :

The problem is that by default matplotlib always defines the x axis increasing, so it will map the points following that rule. Try to reverse it by dong:

ax = plt.gca()
ax.invert_xaxis()

After the plot call.