Builds the horizontal acceleration response spectrum S_a(T) for a chosen seismic design code and set of site/hazard parameters, and plots it against period T. Use the Add to chart button to overlay several codes or parameter sets on the same graph for direct comparison. On the plot, drag across a region to zoom into that period range and double-click to reset the zoom, hover a curve to read its exact T / S_a values, and use the Spectral ordinates button (or its adjacent copy icon) to export the underlying values.