Data index the slider is currently at w.r.t. the Graphic data. With LineGraphic data, the geometry x or y
position is not always the data position, for example if plotting data using np.linspace. Use this to get
the data index of the slider.
Parameters:
graphic (Graphic, optional) – Graphic to get the selected data index from. Default is the parent graphic associated to the slider.
Returns:
data index the slider is currently at, list of int if a Collection