Python plot bar chart and percentage line chart on same graph

I am trying to plot one or more lines on the same chart as a bar chart to show a few different metrics. I heard I should use ax.twinx() for this, but I either get an error saying x and y must have the same first dimension, or a different error reading 0L based on the two things I tried. Here is my code;

    x = df4['Date']
    y = df4['Rate']
    ax = df4[['Date','Qty']].set_index('Date') \
                            .plot(kind='bar',
                                  stacked=False,
                                  color = 'dodgerblue',
                                  figsize=(13,4),
                                  legend=True)
    ax.set_xlabel(r"$\rm \bf{Date}$",
                  fontsize=18, 
                  rotation = 0)
    ax.set_ylabel(r'$\cal \bf{Qty}$ ',fontsize=18, rotation = 90)

    ax2 = ax.twinx()
    ax2.plot(x, y, color = 'green', linestyle = '--', linewidth= 2.0)

Note; df4 is a grouped pandas dataframe. Not sure how relevant that is but just in case.