Hi,
Now I have build a graph like below, showing the distribution of a variable based on its percentiles. Each dot is an observation. Now I wish to show a chart with percentile on the horizontal axis (currently on the vertical axis) and the number of observations on the vertical axis. Can JMP do it without requiring me to create a new "count of observation for each percentile range" variable? Thanks.

This can easily be accomplished by using JMP's ability to add virtual columns. Assuming your data table has a column that has the percentile value, such as below

Then open Graph Builder and drag the percentile column to the X axis

Next, a virtual count variable needs to be created. Right click on the Percentile column and select Formula

Change the value of the formula from the original column( in my example Percentile), to just the number "1"

If you right click on the Transform(percentile) column, you can select "Rename" and change the name to whatever you want. I chose the name "Count"

Close this window and now drag the new virtual column to the Y axis

Now go to the Points control section and change the Summary Statistic to N

And then finally right click on the chart, and select Smoother>Remove

And you will get the final graph

Thanks, Jim, marvelous.
But since many unique percentile numbers correspond to only 1 or 2 observations, the bottom line is too heavy:

What I want is just like the chart below but the dots are not symmetric on Y axis. Is it possible?

Try using the Count virtual column as the Y variable, and change the graph display from Points to Bar
