The height of the line represents the number of values falling within 1/2 of the bin width on either side of the x axis point.
So if the line is at height 200 for an x value of 20 and your binwidth is 10, then there are 200 points from the range of 15 to 25 in the data.
Eventually to be made into an R htmlwidgets library.
ToDo:
Make the bin interval sit as a legend normally but then when moused over it becomes the interval that we use on the y axis.