How to graph mixed data series with total 500k points

I need to draw a graph of the response time of various requests into my webapp. I have parsed the access logs from my application to produce a data file in the following format:

DateTime,ResponseTime,RequestType

For example:

09/Nov/2010:21:00:00,0.002,EVENT
09/Nov/2010:21:00:01,0.040,ACTION

My data file has 500k lines with the different request types all mixed up. The lines are currently ordered by time.

What would be the best tool for drawing all 500k points on one graph with a line per request type, time along the X axis and response time along the Y axis?

In the end I wrote a Java program to parse the data into fewer data points which I could then graph in Excel. The code is specific to my particular use case so I won't bother to paste it in here.