Thanks for sharing. Lots of nice ideas here. A disadvantage of the final plot is that it is very easy to interpret the vertical stacked bar with respect to the vertical scale because they are both vertical and the bar is not labeled with its own scale. Here is a suggestion: why not try the sun-in-the sky approach from your previous plot to show the ratio of nulls in a pie chart? A pie is more naturally interpreted as a ratio of the whole and because it isn't oriented vertically, there is little chance it will be associated with the vertical scale.