I have a dataset that defines the time at which events happen. It does not include timestamps for non-events. I want a 0-1 graph showing 0 on days no event occurred and 1 on days at least 1 event occurred. 

My matplotlib code:

```
    #xs = [a bunch of datetimes]
    #ys = [1,1,.....,1] #of length Xs

    fig, ax = plt.subplots()
    ax.plot_date(xs, ys, '-')

    # format the ticks
    months = MonthLocator()
    days = DayLocator()

    yearsFmt = DateFormatter('%m-%d')
    ax.xaxis.set_major_locator(months)
    ax.xaxis.set_major_formatter(yearsFmt)
    ax.xaxis.set_minor_locator(days)
    ax.autoscale_view()

    ax.grid(True)
    fig.autofmt_xdate()
    plt.savefig('foo.png')

```

This just shows a flat line of `y=1` from the first date in xs to the last date in xs. Probably it is interpolating for the time when there is no Xs values, which are all 1s. 

How can I get `plot_date` to have a y value of 0 for all dates not in xs?