New answers tagged cursor
Something like AutoCAD crosshairs seems to me to be a design which is already well thought out and would work for your situation. You may want to constrain the position of the cursor to specific intervals on the scatter plot. And you could add snapping functionality. I would avoid any situation which requires pixel precise positioning of the cursor.
It widely varies on the type of data visualization you're going to be using. The visualization library d3.js does a good job of switching to an appropriate cursor depending on the visualization. I would dig in their library of visualization and other resources to see if you can find one similar to your situation and how their cursor changed.
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