You can do this in a scatter chart:
- Build a scatter chart with the original series and a range for the x-Axis.
- Copy the first two cells of the x-Axis range to a new range
- Add two y-Values to the right of the new x-Axis cells, make the values equal to the minimum of the original series y-Values.
- Add two new series based on the two new cells – these series will have a single point.
- Format the two new series to have 100% y-Error bars.
- You may have to explicitly set the y-Scale minimum / maximum values
Now you should have two vertical lines (the error bars). Users can move these by clicking on the point which can be on the x-Axis or just above it. The x-Values change on the worksheet when users move one of these points, so it should be easy to do calculations with them.