A previous and now closed discussion suggested a workaround using SkyDrive.
There is a more straightforward solution in Excel than that shown in the Excel Help Forum:
-- Select the chart -- that will bring up 'Chart Tools' on the ribbon above with two tabs: 'Design' and 'Format'.
-- Select 'Design' -- that will bring up 'Add Chart Element' on the far left of a special chart ribbon above.
-- Select 'Add Chart Element' -- that will bring up a sub menu with 'Axis' the top choice
-- Select 'Axis' -- that will bring up three choices: choose 'More Axis Options" -- that will bring up a 'Format Axis' window to the right of the spreadsheet.
-- Select the Paint Bucket icon on the far left -- that will bring up a 'Line' option that might be closed but can be opened up by clicking on the down triangle.
-- You can use the 'Width' drop box to emphasize the line by increasing its size, and 'color' to make it further stand out.