Is your feature request related to a problem? Please describe.
As we are going to proceed on replacing the vislib pie chart with the elastic charts implementation, there is the need to retrieve some data in order the functional tests to work properly.

Data that we need in order the vislib functional tests to run:

• Number of slices
• Slices Labels (array of strings)
• Slices Values (array of strings)
• Get slices colors (array of hex strings).
• Get all legends (and the nested ones) (array of strings)
• I want to be able to click on a specific slice (i.e. filterOnPieSlice('Other')) to test the click event (filtering)
• I want to be able to click on a specific legend action(i.e. filterLegendAction('Other)) to test the action (filtering)
• I want to get the color of a specific slice (i.e. getPieSliceStyle('Other')
• Get the value of a specific slice