It would be nice to be able to select other types of charts such as box plots to add to a Dashboard. Currently, it is only possible to add serial charts or pie charts, which doesn't cover all types of charts sometimes needed to illustrate your data.
In ArcGIS Pro, it is possible to create all sorts of charts: box plot, bar chart, line chart, scatter plot, QQ plot, histogram, profile graph. Currently, in WebApp builder, the chart widget will only let you choose from a bar chart, line chart or pie chart. If you want to display other types of charts, the only way is to create the chart in ArcGIS Pro or Excel or other stats tool and make an image out of the chart to be added in your App. However, it is sometimes essential that the chart is linked to the data or to be able to configure an action.