The type of bar/column chart to render. Possible values are:

• "clustered": Bars are grouped by category. This is the default value.
• "overlaid": Multiple bars are rendered on top of each other by category, with the last series specified on top.
• "stacked": Bars are stacked end to end and grouped by category. Each bar represents the cumulative value of the values beneath it.
• "100%": Bars are stacked end to end, adding up to 100%. Each bar represents the percent that it contributes to the sum of the values for that category.