Add circle to plot (inside of another plot)

figsize=(40,20)
    fig = plt.figure(figsize=figsize)
    ax1 = fig.add_subplot(121)

    # allows to plot only a specific location on the y-z current array (else plot each 'thinning'-squared-th IV curve):
    if coordinate != None:
        ax1.plot(voltages[A:B], pixelplotting_array[:,coordinate[0],coordinate[1]],linewidth = 0.2,color="blue")

        # Create a set of inset Axes: these should fill the bounding box allocated to them.
        ax1_2 = plt.axes([0,0,1,1])
        # the best looking fontsize is about half of the width of the figsize
        fontsize = figsize[0]//2

        ax1_2.imshow(current_array, interpolation='nearest', cmap=cm.afmhot, origin='lower')

        # Manually set the position and relative size of the inset axes within ax1
        ip = InsetPosition(ax1, [0.2,0.2,0.3,0.3])
        ax1_2.set_axes_locator(ip)
        # Mark the region of the solar cell where the shown IV curve emanates from.
        circle = plt.Circle((0.5, 0.5), 0.4, color='blue')
        ax1_2.add_artist(circle)