SVG code for individual frames was automatically generated by the "Scatterplots" spreadsheet linked at User:RCraig09/Excel to XML for SVG. Additions and adjustments were made in a text editor.
Inkscape imported the SVG, adjusted opacity of red and blue trendlines (each: 15%, 30%, 50%, 100% opacity), and exported to form four blue-trendline PNGs and four red-trendline PNGs. One further PNG has no trendlines.
Gimp imported PNGs as layers, and created looping animated GIF (70 ms/frame) containing: 31 no-trendline frames, three red-trendline frames ramping up to full (22 frame) red-trendline frames and three red-trendline frames ramping down to three no-trendline frames, three blue-trendline frames ramping up to full (22 frame) blue-trendline frames and closing with three blue-trendline frames ramping down. Total: 90 frames.
Note that the baseline time period changed to the pre-industrial value, so the trace is shifted up on the vertical axis temperature scale, compared to earlier versions of this chart.
Version 4: Shortened length of red bars to be commensurate with time periods over which they estimate. . . Squaring off ends of red and blue bars (formal) . . . Increasing frame duration from 60 to 70 ms . . . increasing duration of no-bars graphic and reducing duration of red- and blue-bar graphics