More exploration of this topic:
For those who would like to automatically generate a goal or constant line, you can use Greg's method. However, in an autogenerated series, your goal or constant line will have the same properties as the other lines in the series. So, for example, if you have two series lines with markers and labels, your third "goal line" series will also have markers and labels. For my goal lines, I just want them to be a plain, straight red line with no markers or labels.
You can also create a non-autogenerated series in the Chart Wizard that would read your goal line inputs (dates, constant goal value) from a second query in the Data tab of your report file, and you can customize it to be just a plain, straight red line with no markers or labels, for example. The only drawback to this, for me, is that the name of the series which is displayed in the legend of the chart has to be hard coded in the Options tab for the series. I.e. I can label the goal line "Goal" for all my reports, but I can't have "Goal = 98%" in one, and then "Goal = 90%" in another without creating two different report files. Based on how we present data, it's important to have the Goal % displayed in the legend.
It appears you can only have one autogenerated series per chart, otherwise this could be solved by adding a second autogenerated series.