2 possibilities:
1) If possible, create your runtime value as a formula and then add it as an extra field to be charted. It will show up as a bar, but you can click on the bar in the PREVIEW mode view and bring up right mouse menu "Series Options". Here you can change ("emphasize") the bar into a line.
2) Our add-in library, CRChart, has extensive support for user-defined lines and allows them to be set at runtime. It also puts the value on the line and some custom-user text (ie "Average: XXX.XX"). But it DOES cost money.
-Dan
disclaimer: I work for the company, threedgraphics.com, that makes CRChart.