Can anyone tell me what is the name of this type of diagram ( if any )? Also can anyone suggest any tools, however simple, to plot such a diagram?
If the difference between tree plot and mosaic plot coincides with the distinction between hierarchically-arranged categories and how single categories are broken down, then OP's image would appear to be a tree plot.
I'll research this issue further when I have a moment.
Treemapping is an information visualization method displaying hierarchical data by using nested rectangles.
It has origins in mosaic plots (left) or Marimekko charts (right), adding nesting or embedding to the standard mosaic structure. The one you displayed is well-balanced, without elongated, skinny rectangles, that degrade the appearance of some treemaps generated by the "slice-and-dice" tiling algorithm.
So it belongs to the subspecies of "squarified treemaps", that can be decorated with colors or shading in cushion treemaps. A description is given in Mark Bruls et al. (2000) Squarified Treemaps, Proceedings of the Joint EUROGRAPHICS and IEEE TCVG Symposium on Visualization in Amsterdam, The Netherlands, May 29–30, 2000. Apparently, the idea of squarifying, i. e. constraining rectangles to low aspect ratio, was already present in M. Zizi M. and M. Beaudouin-Lafon (1994). Accessing Hyperdocuments Through Interactive Dynamic Maps, Proceedings of the 1994 ACM European conference on Hypermedia technology.