You actually have it almost complete.
Create 2 new variables, exactly as you have them:
1.05* (9158 + (.0903 * [Volume]))
.95* (9158 + (.0903 * [Volume]))
Then to display on the graph, put them on the dual axis. (Easiest method, ctrl-select both fields, drag to the right axis, drop when you see a plus sign. You can also pull them onto the Rows tab and select Dual Axis.)
Then synchronize the vertical axes: right click on the right axis, hit Edit Axis, and check "Sychronize Dual Axes".
Now the trickier part: It will still have symbols on the line because of the primary axis settings. To remove, under the Marks page menu (down arrow in upper-right corner of that page), select Multiple Mark Types. This allows you to define the primary and secondary axis differently. Select mark Measure Values, and this is where you change the appearance of these new trend lines. Now what you want to do is only show the trend line, and not the values (because you only have values when you had volume). There are several ways to do this, but what I'd do is make the symbols 100% transparent. Pull down the drop-down box in the Color pill, and pull the Transparency tab to 0%. Now all you see are those trend lines, and it should be good to go.
Hope this helps, let me know if you have any questions!
Perfect! Thanks for taking the time to help - much appreciated!!