[DOC] architecture doc about label bounds with mxgraph

docs/architecture/mxgraph-integration.adoc

@@ -87,3 +87,56 @@ waypoints of BPMNEdge are always relative to that plane’s origin point and are
 [quote]
 The bounds of BPMNLabel are always relative to the containing plane’s origin point. Note that the bounds’ x and y
 coordinates are the position of the upper left corner of the label (relative to the upper left corner of the plane).
+
+
+==== Label Font and Bounds
+
+The `mxGraph` integration renders labels based on the definition taken from the BPMN source. When the label style is not
+or only partially defined in BPMN, the following fallback takes place:
+
+* for labels without a font, the rendering may not be accurate: the default font used by the original modeler may differ
+from the one the lib uses. As a result, the bounds set in the BPMN file may be too small or large for the lib font. So the
+resulting text wrapping cannot be the same as with the original modeler.
+* for labels without bounds, the lib uses an arbitrary position which depends on the type of the BPMN element (for instance, middle
+centred for tasks or on the bottom for start events). As for fonts, this does not cover all use cases, but works most of
+the time.
+
+===== Shape
+
+The `mxGraph` integration uses the following to set the label bounds
+
+* the `vertex mxCell` geometry offset, see https://github.com/jgraph/mxgraph2/blob/mxgraph-4_1_1/javascript/src/js/model/mxGeometry.js#L60[mxGeometry]
+for more details
+** x and y position relative to the shape `vertex` itself
+** as for the  shape `vertex` position, a coordinate transformation layer is required.
+* styling for the label width (for word wrapping)
+
+===== Edge
+
+====== Overview
+
+The mxGraph integration uses the `edge mxCell` geometry to set the label bounds
+
+* absolute geometry
+* offset x and y: position related to the center of the edge and as for the Shape Position, a coordinate transformation layer is required.
+** the center depends on the 2 terminal waypoints
+** if the waypoints are not available, no position is set and the label is placed on the edge center
+* width and height: for word wrapping.
+
+see https://github.com/jgraph/mxgraph2/blob/mxgraph-4_1_1/javascript/src/js/model/mxGeometry.js#L60[mxGeometry]
+
+====== mxGraph Details
+
+The label position is related to the 'center' of the edge, see https://github.com/jgraph/mxgraph2/blob/mxgraph-4_1_1/javascript/src/js/model/mxGeometry.js#L35[mxGeometry]
+for more details. The definition of 'center' differs whether the `mxGeometry` is `relative` or not:
+
+* if relative, it is the center along the line
+* if absolute, it is derived from the terminal points
+
+This is explained in https://github.com/jgraph/mxgraph2/blob/mxgraph-4_1_1/javascript/src/js/view/mxGraphView.js#L2187[mxGraphView.updateEdgeLabelOffset]
+
+* center between the two endpoints if the geometry is absolute
+* the relative distance between the center along the line, and the absolute orthogonal distance if the geometry is relative.
+
+Check the https://github.com/process-analytics/bpmn-visualization-js/pull/291#issuecomment-642024601[GitHub Pull Request #291]
+to see various positioning methods in action.