Hi Dave,
The defaults for marker style are in ISO/IEC 29500-1 section 21.2.3.46 ST_Style (Style). They are determined by the value of “val” from the <c:style val=”2”> element. Table 1 shows chart default styles. Some of them have defaults for all style values others use the tables in the section to determine their style from “val”.
For “Fills for Data Points (2-D)” the defaults come from Table 5. Looking at the Fill, for example, in table 5 for val=”2” the fill is 2. Table 3 has the default fill formatting per chart style, for 2 in Table 3 the color is bg1.
There are elements besides the scatter that have markers by default, e.g. Line with Markers and Radar with Markers also have markers.
Let me know if that answers your questions.
Best,
-Mike