What does a dashed line on a solubility curve represent?
The dashed lines are line styles that distinguish the solubility curves of different substances.
Chemistry to cram as many solubility curves as possible into a single graph.
For example, the graph below contains a dozen curves.
Six of the graphs use solid lines. Five use dashed lines. And KCl uses alternating dots and dashes.
If the designer had used solid lines for everything, the graph would have been a confusing mess.
The different line styles make it easy to follow one solubility curve over the temperature range.
The video below uses solubility curves with dashed lines.