Graph in LibreOffice Calc does not render straight line

Plotting 1145 rows GPS data in scatterXY graph type does not render me a straight line.
Instead I have a graph filled with a kind of irregular bullets.
When I hide anywhere a 100 lines in my data, then the outcome is a solid line as expected.
Is there a limitation in rows??? Any idea for a solution ?
(sorry as newcommer on this forum i do not have points for uploading images to illustrate)