Four line MSChart: how can I add filled circles as data markers?

Why doesn't my VtChChartType2dXY chart show any data markers. I have 10 data points on each of 4 smooth, slightly curved visible lines, but markers don't show.

Code:

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With oOut.MSChart1.PLOT
.SeriesCollection(JJJ).DataPoints(-1).Marker.Size = 50
.SeriesCollection(JJJ).DataPoints(-1).Marker.Style = VtMarkerStyleFilledCircle
.SeriesCollection(JJJ).DataPoints(-1).Marker.Visible = True
End With

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JJJ values are 1,3,5,7 corresponding to the x columns of the data array.

Q1 Is my syntax correct?

Q2 Is there a webpage showing all possible formatting words available for line and scatter plots?

Q3 What are the ways to adjust the horizontal and vertical gridlines, like width, dashed, dots, etc.?