Allow users to fix y-axis

[Mdraugelis]

To enable reporters to compare scenarios they need to be able to fix the y and x axis.

[nickcanz]

@Mdraugelis could you provide a little more detail? I'm guessing this means the ability to set a max Y value to a number that they input. Should it be one input that goes across all the graphs?

[quinn-dougherty]

@nickcanz i think setting max y value makes sense.

please add an int to the chart function input that's defaulted None, and if it isn't None when you .encode( pass in y=alt.Y('<column>', scale=alt.Scale(max=<int> something like this