markers

Two things

By markers I'm assuming you mean data points in a series.

First, did you place the chart on a separate sheet? Sometimes increasing the chart area to a full sheet can help make things more clear.

Also, there is no way (to my knowledge) to tell excel to ignore points that you have selected for a chart, the trick is to select less points!

One basic data reduction is to take out every other data point. Say you have X data in column A and Y data in column B. There are definitely better ways to do this (like with VB script) but here's the way I took out half of the data points in the series. In C1 type the formula:
=IF(ISODD(ROW(B1))=TRUE,B1," ")
note the space between the second set of quotes, for this admittedly hack answer to work you'll need that space
If you copy the formula down you'll see values only for the odd numbered rows, but if you try an X-Y graph based on col(A) vs col(C) you will find that the "blank" cells (they are not truly blank) are treated as 0's and plotted accordingly.

So copy the C column and Edit-->paste special, "as values" into D column. Then highlight the D column, go to Edit-->Replace and in the "find" box, input a single space, make sure the "replace with" box is empty, and click the "replace all" button.

Now graphing Column A vs Column D should give you half of the data points present in Column A vs Column B.

Some of the scripting pros on this board could also give you a vb script that would just place the values where you need them in col(C) without the need for the formulas, "paste as values", and "find and replace" procedures.

Hope that helps