我想在matplotlib/pyplot中的图像文件上绘制一个透明的等值线图。
这是我目前得到的。
我有一个600 x600像素的正方形图像文件test.png，看起来像这样：

我想用matplotlib和pyplot在这个图像上绘制一个等值线图（让图像文件在“下面”，并覆盖一个半透明版本的等值线图）。作为一个额外的好处，图像会自动缩放以适应当前的绘制边界。我的示例绘制脚本如下：

from matplotlib import pyplot
from matplotlib.ticker import MultipleLocator, FormatStrFormatter
from matplotlib.colors import BoundaryNorm
from matplotlib.ticker import MaxNLocator
from pylab import *
import numpy as np
import random

# ----------------------------- #

dx, dy = 500.0, 500.0
y, x = np.mgrid[slice(-2500.0, 2500.0 + dy, dy),slice(-2500.0, 2500.0 + dx, dx)]

z = []
for i in x:
    z.append([])
    for j in y:
        z[-1].append(random.uniform(80.0,100.0))

# ----------------------------- #

plot_aspect = 1.2
plot_height = 10.0
plot_width = int(plot_height*plot_aspect)

# ----------------------------- #

pyplot.figure(figsize=(plot_width, plot_height), dpi=100)
pyplot.subplots_adjust(left=0.10, right=1.00, top=0.90, bottom=0.06, hspace=0.30)
subplot1 = pyplot.subplot(111)

# ----------------------------- #

cbar_max = 100.0
cbar_min = 80.0
cbar_step = 1.0
cbar_num_colors = 200
cbar_num_format = "%d"

# ----------

levels = MaxNLocator(nbins=cbar_num_colors).tick_values(cbar_min, cbar_max)
cmap = pyplot.get_cmap('jet')
norm = BoundaryNorm(levels, ncolors=cmap.N, clip=True)
pp = pyplot.contourf(x,y,z,levels=levels,cmap=cmap)
cbar = pyplot.colorbar(pp, orientation='vertical', ticks=np.arange(cbar_min, cbar_max+cbar_step, cbar_step), format=cbar_num_format)
cbar.ax.set_ylabel('Color Scale [unit]', fontsize = 16, weight="bold")

# ----------

CS = pyplot.contour(x,y,z, alpha=0.5)

# ----------

majorLocator1   = MultipleLocator(500)
majorFormatter1 = FormatStrFormatter('%d')
minorLocator1   = MultipleLocator(250)

subplot1.xaxis.set_major_locator(majorLocator1)
subplot1.xaxis.set_major_formatter(majorFormatter1)
subplot1.xaxis.set_minor_locator(minorLocator1)

pyplot.xticks(fontsize = 16)
pyplot.xlim(-2500.0,2500.0)

# ----------

majorLocator2   = MultipleLocator(500)
majorFormatter2 = FormatStrFormatter('%d')
minorLocator2   = MultipleLocator(250)

subplot1.yaxis.set_major_locator(majorLocator2)
subplot1.yaxis.set_major_formatter(majorFormatter2)
subplot1.yaxis.set_minor_locator(minorLocator2)

pyplot.yticks(fontsize = 16)
pyplot.ylim(-2500.0,2500.0)

# ----------

subplot1.xaxis.grid()
subplot1.yaxis.grid()

# ----------

subplot1.axes.set_aspect('equal')

# ----------

pyplot.suptitle('Main Title', fontsize = 24, weight="bold")

# ----------

pyplot.xlabel('X [m]', fontsize=16, weight="bold")
pyplot.ylabel('Y [m]', fontsize=16, weight="bold")

# ----------

implot = subplot1.imshow( pyplot.imread('test.png') , interpolation='nearest', alpha=0.5)

# ----------
pyplot.show()
#pyplot.savefig("tmp.png", dpi=100)
pyplot.close()

...但我没有得到想要的结果...相反，我只看到了轮廓图部分。类似于：

我应该在代码中做些什么来得到我想要的东西？