fix linting error in plots

[marjoleinpnnl]

No description provided.

[djinnome]

Why does changing the name fix the linting error?

[marjoleinpnnl]

Joseph figured it out. Since we had used png_data elsewhere, and it was a bytes argument then, it expected it to be a bytes (and not string) result again the second time. But svg_data is a string.

[JosephCottam]

Why does changing the name fix the linting error?

The linter wants a variable to have the same type everywhere in the function/method. png_data was used earlier to hold png data (bytes), the SVG code was built via copy-paste from that branch of the if statement. Even though png_data in the SVG branch only ever held an str and even though the png and SVG branches can never both be executed, it wants the variable name to be have that consistent type. I had missed this in my prior attempts to fix it, but once I saw @marjoleinpnnl's encode based solution, it made it clear what was going on.