Create multiple waterfall bar charts

### Feature Proposal

Currently we can create waterfall chartjs having only one bar chart. But what if the user needs multiple bars stacked upon one another in the form of waterfall chart.

### Possible Implementation

The problem can be solved by using an array of arrays. Currently to show waterfall chart we have to specify an array like [0,20] which means the bar will go from 0 to 20 points vertically. Similarly we can create other arrays which will form the base bars of the waterfall graph. They will look something like this A = [[0,20],[20,40],[40,60]] and then we can directly have another array containing the height of the bars that are need to shown above the 3 bars in array A and so on.