Horizontal trapezoid needs two sides parallel to x-axis. Points with same y-coordinate form a horizontal line. Group points by y — each group gives candidate horizontal edges.

For y-level with v points, number of horizontal edges = v*(v-1)/2. Count trapezoids by pairing edges from different y-levels: accumulate product of edge counts between current y-level and all previous y-levels.

Order of y-levels processed doesn't matter — running sum of previous edge counts gives all pairs exactly once. Modulo 1e9+7. Single y-level insufficient (need two distinct y-levels for two horizontal sides).