I trust that you're using an XY chart, not a line chart, for this exercise.
One approach, if you want to manually draw your own "bets" line, would be to
add an XY series with two points, the first point being at the point of the
first series where you want the tangent. Select the other point of this
added series (two single clicks: one to select the series, one to select the
point). When the cursor goes over the point, it becomes a four-headed arrow.
Now you can click on the point and drag it horizontally or vertically, until
it lines up the way you'd like. Then add a trendline to this series and pick
the slope off of the regression formula (adjust the number format of the
formula so you have lots of digits), or use LINEST on the worksheet data for
this series (the data in the cells will change as you drag the point).
This technique works in all Excel versions since before Excel 97, but it
will no longer work in Excel 2007, as Microsoft is removing the ability to
drag a point with the mouse.
- Jon
