This plugin is a fork of d3-zoom that adds several features by separating scaling on x and y:
Scale independently along x-axis and y-axis
Constrain scale extent on zoom out to respect translate extent constraints
Apply "scale factor ratio" on user input. For instance, with a scale ratio of 0.5 on x-axis and 1 on y-axis, when user zoom with its mouse, the increase of scale factor on x-axis will only be half of the increase on y-axis.