Histogram with Benchmark, adding a point to show where Curve meets benchmark.

Short but useless answer -- yes, I'm sure there is a way to add a single data point.

So much of charting seems to be about the way you arrange data in the spreadsheet. Where are you putting the data point for the 3rd data series? Where are the data for the other 2 data series? My first expectation would be something like:

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Put the data point in the row where you want the benchmark point to be. This third series should be added as a line chart type and should probably be formatted as markers without lines.

I had assumed that the single data point would be directly above one of the columns.

An even scale is the standard behavior for a category axis (the kind of axis used by line and column charts). An axis that takes into account the numerical value of its data point is a value axis. The only chart types that support a value horizontal axis is a bubble or XY scatter chart. So, this single point data series will need to be added as an XY scatter chart type -- on top of the column + line combination chart you already have. Assuming you have the column and line data series in three columns as indicated:

1) In a convenient place in the spreadsheet, enter your X value (0.673) and Y value (6:00) for this single data point.
2) Add a new series to the chart and change its chart type to XY scatter. (In my copy of Excel, Excel automatically adds the secondary axis system to the chart. If your copy does not automatically add these axes, you will probably want to add them.).
3) Edit the series so that the X values points to the X value cell and the Y values field points to the Y value cell.
4) Now comes the tedious part -- figuring out the secondary horizontal axis and/or X value to position the single data point correctly.
4a) If this is a one time or rare task, I would probably just format the secondary axis so that the min and max values for the axis position the data point correctly.
4b) If this will be a frequent task, then I would spend time with the secondary axis and/or X value so that I can fix the axis limits at some convenient values (0 and 1 or 0 and 100 or whatever makes good sense to me), then compute a "dummy" X value for this point using the number of categories in the column+line chart and the position I want.
5) Format the secondary Y axis so it matches the primary Y axis. Hide the secondary axes, if desired.