how to stop pyplot from reversing the values in my x-axis

Question

Signal is 2048 long integer array. The plot is shown below.

See that the velocity axis has negative values on left and positive values on the right. When I created the velocity array it starts with positive numbers and ends with negative numbers, why does the x axis reverse the plot? how do I get it to plot left to right instead of right to left? (positive to negative instead of negative to positive)

thanks

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import numpy.polynomial.polynomial as poly
from scipy import asarray as ar,exp, optimize
import numpy as np
import matplotlib.pyplot as plt
from scipy.optimize import curve_fit

signal = np.genfromtxt("rawdata/g924310/r_g924310_b1.txt")

for i in range(1,2048,1):
    vel[i] = (1024.5 - i)* 0.137177
signal = np.genfromtxt("rawdata/g924310/r_g924310_b1.txt")

plt.plot(vel,signal,'b+:',label='scan')
plt.legend()
plt.title('1667 MHz (92.4, -31.0) - Antenna Temp v Vel')
plt.xlabel('Velocity')
plt.ylabel('Antenna Temp (K)')
plt.show()

Answer 1

By default matplotlib will order axes from smallest to largest. You can override that default for the x-axis by calling invert_xaxis on the Axes object:

from matplotlib import pyplot as plt
import numpy as np
x = np.array([3, 2, 1])
y = np.array([4, 5, 6])
plt.plot(x, y)
ax = plt.gca()
ax.invert_xaxis()
plt.show()

Answer 2

You can also say plt.xlim(vel[0], vel[-1]) to directly force your array first and last elements to be the start and end points of your plot.