First, it would be nice if you can have multiple representations of the same variable in the tooltip. For example; If I am measuring liquid level in a cylinder it would be nice to plot the level in feet on the line chart, but when you hover over the data point it says the level in ft, and also the volume in gallons or whatever else you'd like.  You could do this by taking in that same variable and applying a formula to show the formulated value in the tooltip of both ft and gallons.

Second, it would be nice if the data points are from the same group or time so that the tooltip would group them across multiple variables within the line graph like the photo attached.