How can I interpret this solubility data?
Yes, the website is a little bit ambiguous. However, there is a clue in the colour legend. The solubilities are indicated in units of grams per 100 grams of water. (This is also a very common unit for expressing solubilities.) Therefore, a solubility of "$3.7$" means that $3.7~\mathrm{g}$ of $\ce{KNO3}$ dissolves in $100~\mathrm{g}$ of whatever solvent it is listed under. This interpretation is readily confirmed by checking the values against this Wikipedia page.

As for "$50\%$", my educated guess is that it refers to a mixture of $50\%$ ethanol and $50\%$ water.

And lastly "$20^\circ$" refers to $20~^\circ\mathrm{C}$.