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If you're a CC subscriber you can copy it into Illustrator and it'll paste as a vector (assuming it's a proper chart and not a placed screen grab like ~60% of the authors I work with prefer 🙄), then you can do what you want with it. Not relevant in this case but text doesn't usually survive the transfer, so don't get your hopes up if you require a bit more detail in future attempts.
How do I export just the line in this exact position from powerpoint? I.e. transparent png/whatev with just a red line running through it.
I've tried changing the plot and chart area to no fill.
Double clicking the background just offers solid/gradient/picture/pattern. If I hit solid for example and assign a colour to it it's no good, dragging the transparency toggle to 100 just reverts it to the attached with white bg.