I don't think you can control where the reference label will appear relative to the line/bar on a continuous X axis reference. You can try adding the desired label as Text in Chart (AGGR will probably be needed here), and using CTRL+SHIFT+Drag to put it at the top left of the chart. This will not line up with a specific bar, though - I don't know of any way to get that done. You may prefer to add a Y-axis reference line (set to max value with AGGR) and label that line.
