This visualization shows each pass of the mergesort algorithm as a horizontal row of 200 vertical line segments, with each row representing the array state after one merge phase. The lines are rotated according to their values, creating a waveform-like pattern that reveals the sorting progress from top to bottom. The implementation uses D3 v3 with ordinal and linear scales to position and rotate the lines, and SVG line elements are rendered within separate SVG groups per row. The mergesort function records each intermediate array state, and the code uses `d3.shuffle` and `d3.range` to generate the initial unsorted data.