Linear line on Scatter Chart but define the slope and y-intercept

[#1]

Is there a way to plot a linear reference line on a scatter chart where I can define the slope (m) and intercept (b) using the line equation y = mx + b?  See the screenshot below of my chart in QV.  I've added a linear trendline expression (the red line), but the equation shown isn't quite what I'm looking for.  I need something like the blue line that I've drawn in.  This line needs to cross where the averages of x and y meet and have a slope m = -1.   I've tried playing around with adding a third reference line, but I can't seem to get anything but perfectly horizontal or vertical lines.

PS. I only have QV PE here at work, so please don't attach examples in qvw.