How this calculator works
A frequency polygon places each class count at its midpoint and connects adjacent points. This tool adds zero-height points one class spacing before and after the supplied classes to close the polygon. Grouped input requires increasing, equally spaced midpoints and whole non-negative counts. The output is a coordinate table; no interpolated probabilities are implied between points.
Formula
midpoint = (lower edge+upper edge)/2; relative height = class count/total; closing x values = first midpoint-width and last midpoint+width.
Worked example
Midpoints 1, 3, 5 with counts 2, 3, 1 give points (-1,0), (1,2), (3,3), (5,1), (7,0). The total count is 6.
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Frequently asked questions
Why add zeros outside the classes?
They close the conventional polygon at the baseline without altering the observed counts.
Can class spacing vary?
This tool requires equal spacing so heights and closing points have a consistent interpretation.
Does the table draw the connecting lines?
It supplies their vertices. Connect the coordinates in order in a plotting tool.