Scatterplot with Categorical Date x axis NO AGGREGATION

Without getting into fleshing out your semantic model, etc., you could just add a calculated column to convert your dates to integers. Then, if you use those date integers in the x-axis, you can use "Don't summarize" on the field you want to split up on each tick rather than aggregate. And also it should be trivial to toggle on the trendline (if I'm understanding what you are trying to do there).

Example with your data.

Sample

Calculated column

Day Num = CALCULATE( MIN( 'Sample'[Order Posted Date] ) ) - MIN( 'Sample'[Order Posted Date] )

And here is scatter chart with field wells - toggled trendline on without any additional action

And, to get the above, I set up some dynamic axis ranges so it looks little better (this is snip of format pane)

xmax = MAX( 'Sample'[Day Num] )+1

ymin = MIN( 0, MIN( 'Sample'[OrderTAT] ) * 1.1 )

ymax = MAX( 'Sample'[OrderTAT] ) * 1.1

(and, as you can see from snip, just set xmin to -1 in pane as the DAX in calculated column ensures min is always 0 so by extension -1 is always a good buffer