I had a similar question here, and the answer was to use a dirty hack. Matplotlib histogram with collection bin for high values

So with the following code, you get the ugly histogram you already have.

def plot_histogram_04():
    limit1, limit2 = 50, 550
    binwidth1, binwidth2 = 10, 50    
    data = np.hstack((np.random.rand(1000) * limit1, np.random.rand(100) * limit2))

    bins = range(0, limit1, binwidth1) + range(limit1, limit2, binwidth2)

    plt.subplots(1, 1)
    plt.hist(data, bins=bins)
    plt.savefig('my_plot_04.png')
    plt.close()

In order to make the bins equal width, you indeed have to make them equal width! This means manipulating your data such that they all fall in bins with equal width, and then play around with the xlabel.

def plot_histogram_05():
    limit1, limit2 = 50, 550
    binwidth1, binwidth2 = 10, 50

    data = np.hstack((np.random.rand(1000) * limit1, np.random.rand(100) * limit2))

    orig_bins = range(0, limit1, binwidth1) + range(limit1, limit2 + binwidth2, binwidth2)
    data = [(i - limit1) / (binwidth2 / binwidth1) + limit1 
            if i >= limit1 else i for i in data]
    bins = range(0, limit2 / (binwidth2 / binwidth1) + limit1, binwidth1)

    _, ax = plt.subplots(1, 1)
    plt.hist(data, bins=bins)

    xlabels = np.array(orig_bins, dtype='|S3')
    N_labels = len(xlabels)
    print xlabels
    print bins
    plt.xlim([0, bins[-1]])
    plt.xticks(binwidth1 * np.arange(N_labels))
    ax.set_xticklabels(xlabels)

    plt.savefig('my_plot_05.png')
    plt.close()