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You can specify multiple locations, although there is no particular advantage to doing so. The software will simply take the median x-value of the selections and use that as an initial guess for the trace finding code, which fits a polynomial for trace x-position as a function of y-pixel. Once you have selected your object, press 'q' or click 'Done' and the code will trace your selection. When it has finished, it will plot up the results:
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