I'm not sure if you'd tried this or if this will work for your setup,
but I tried using a dual axis, the second y-axis
being the same field as the x-axis.
I then set the transparency of the second axis to 0%
and just showed its trendline.
Of note, just to make the second axis have less of
an impact on the visualization, I removed its tooltip text
and set its size to the smallest possible.
Please see this link for another example:
211589slope.twbx 1.5 MB