Adding new data to excel graph with different horizontal axis data

[StephenSLR]

I would like to compare temps and values of two sets of data with a standard.

I have added in both sets of data for C (Horizontal axis) for Hg and V.

I would like to add in new data (in green) to create a line on how Hg and V compare to the R (which is a standard) to show how far off both sets of data is to the standard.

The only issue is, I have no values at the lower temps (vertical axis) so I'd like the new line to be a half line that appears on the graph, starting half way across the graph but it will correspond to different horizontal axis values.

Also how do I place in a dot point on the graph for that first line of data?

Here is what I have so far:

Thanks for looking.

s