Feature Request: Mekko Bar

## Mekko charts
Mekko charts are a very visually appealing alternative to scatterplots when the number of observations is small.

## Rationale
Although LetsPlot certainly has the tools to create them, the number of data transformations needed might make Mekko Bars a good addition to the Bistro Plots module.

## Expected inputs
Mekko charts take three inputs:
- Labels to use for the X axis.
- An X axis metric which needs to be accumulated up to the plot size. / Alternatively, the number of entries of the label is counted.
- An Y axis metric which behaves similar to how a bar chart would.

## Examples

### Q1 profit vs Q1 profit growth

### Investment/Number of deals vs Average deal size

### Total GDP vs Debt (% of GDP)

[Chart source](https://www.mekkographics.com/product/power-of-mekko-charts/)