Draw an SVG Element on the Leaflet World Map

This draws an svg circle into the SVG layer at zoom level 1. Its center point is lat/lng(0,0), x/y(400,200). It has a requested radius of 800 km. Before the circle can be drawn, the requested radius value must be converted to svg pixels. During map zoom level changes, the circle's center is moved to maintain its location. It is not scaled, as the symbols are, but keeps it actual size(radius=800 km) during all map zoom levels. Therefore, this requires that its pixel radius must be re-computed(scaled) at each map zoom level change.

1). The map is placed into its DIV (width:800px, height:400px).
2). The map is centered at Lat/Lng (0,0) at zoom level 1.
3.) The SVG element is added via initPathRoot, creating the SVG layer
4.) An SVG rect background is added, along with circles at its center and corners.
4a.) The rect and circles show the SVG layer extent at initial loading.
6.) The svg viewBox is computed and shown. This SVG layer nicely covers the world map
7.) As the cursor moves over the SVG layer, its x,y values are shown. Uses svg methods createSVGPoint, getScreenCTM, and matrixTransform
8.) The cursor x,y values are then used to compute the associate map Lat/Lng values,via the map method layerPointToLatLng.
9.) A line is attached to the map lat/lng(0,0) and follows the cursor. Its pixel length and map meters length are computed.
Map distance is measured by method latLng1.distanceTo(latLng2).
10.) When the map zoom level is change the line's x1,y1 location is changed.
11.) An SVG circle is initially drawn with its center at lat/lng(0,0). Using the requested radius(800km), the radius in pixels must then be computed. A 'target' lat/lng point is determined that is at a distance of 800 km from the center lat/lng. The radius in pixels is then determined via latLngToLayerPoint(targetLatLng).
12.) When the map is zoomed, the center point is adjusted and the circle is scaled maintaining, the radius of 800 km.
VB x:VB y:VB w:VB h:
Cursor Location:   SVG x: SVG y:    Map lat: Map lng:
Line Length:   SVG pixels:    Map km

SVG Source: Javascript:
OK in:IE11/CH40/FF35