Holding Shift does snap to 45 degree angles, including horizontal.

To draw on a chart sheet, select the chart as well as switching to the
sheet.

BUT drawing over you chart this way is pretty clunky - what if your
chart is updated, your data changes and the axis rescales, all those
sorts of forseeable events?
Why not plot an extra series with two points to draw the line for you?
Make it a line chart, move it to the secondary axis if necessary and
even 'strecth' it to the sides of the plot area (by adding then
formatting the secondary horizontal axis to show vertical axes at the
tick marks instead of between them)

Hope this helps
Adam

On 10/03/2010 14:56, ExcelGeek75 wrote:
Help! I need to draw a line on a chart that is horizontal - - in old Excel,
the SHIFT key could be used to "snap to" the horizontal, but in Excel 2007,
it does not work and it is driving me crazy!