Unstacked:In this visualization mode, every measure is plotted against the Y-axis, but the scale is not fixed, it will be changed when you focus the measure to highlighted in the graphic.
Shared Y-axis: It means every measure is plotted against one Y-axis, and the scale for Y-axis is fixed, it would not change. For example, if you are comparing the temperature of two device over the past day, you can create a chart that shows the temperature for each device on a single chart with the same X and Y axes.
The main difference between Shared and Unstacked mode is the scale of Y-axis, one is fixed while another one is dynamic.You can compare the difference from the following image.
