Plotting curves showing less points than actually used

Yes, this can be done in Excel. Here's I usually see this sort of thing happening:

1) Decide on a form the best fit curve, or decide how you want to interpolate/extrapolate. This is probably the hardest part of a question like this, and it really isn't an Excel question, or a question specific to the programming language or application you are using. You have to know enough about your data and the math behind the curve you want to draw to decide what those curves should look like. Without more information, I'm not sure I can provide much more input on this step.

2) Once you have decided how you want to interpolate/extrapolate/curve fit the data, then you can program the regression/interpolation/extrapolation into Excel at any number of points over whatever range you want in another table.

3) Then plot the curves and format as desired

In that case, I would start with this (not really Excel specific) question: How will you determine which points to plot? For example, If you have 10,000 sorted, somewhat evenly spaced data points, it might be easy enough to plot every 1000th data point.

Once you've decided which data points you want to plot, use Excel's lookup functions and extract those data points from the main table into another table. Then use this table to plot the series that will be for the markers.