Allow two series to be on different axes, but have the same scale.
Scales are auto-computed and are likely different, depending on the data.
Allow a setting to force a series scale to match the scale of another series.
Consider a stacked bar chart overlayed with an unstacked line chart. The bar chart needs to be stacked while the line series does not. An example is a production facility producing multiple parts where each part is represented by a stacked bar. The facility capacity is represented by an unstacked line (or a stacked line, but stacked separately from the bars). Here we need two y-axes, but we want the same exact scale so that we can display the production quantities vs the production capacity on the same scale.