* getredash/redash#2629 Refactor Chart visualization, add option for handling NULL values (keep/convert to 0.0)
* Handle null values in line/area stacking code; some cleanup
* Handle edge case: line/area stacking when last value of one of series is missing
* Mjnor update to line/area stacking code
* Fix line/area normalize to percents feature
* Unit tests
* Refine tests; add tests for prepareLayout function
* Tests for prepareData (heatmap) function
* Tests for prepareData (pie) function
* Tests for prepareData (bar, line, area) function
* Tests for prepareData (scatter, bubble) function
* Tests for prepareData (box) function
* Remove unused file
