You need to use tables with 2 rows. One for the caption text and one for the diagram object.
The table can be pinned to the page so that it does not move but text will flow around it in the way you wish.
Table properties
Text Wrapping = around
Positioning
Horizontal and vertical positioning = Relative to page
Move with text = off
You can drag the table to where you want it on the page so that it aligns with the margins/headers. Alternatively you can type numbers into the boxes that appear to give only the options for left/righ/top/bottom etc.
I've attached an example table.
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