How to group/bin data based on row and column values and plot result?

I have a data set that is recorded engine data over time. I would like to make a bubble plot of percent occurence that a given variable spends at a given engine RPM (i.e. x: RPM, y: Pressure, z: precent occurrence of pressure at given RPM). The attached plot shows what I am trying to acheive. For example, at 2000-2500 RPM, the pressure occurs at 2000-2500 hPa 18.4% of the recorded data.

I have found and used bubbleplot to acheive plotting requirements. However, I am stumped on what functions that I could use to group/bin the data by X and Y

Any help is appreciated

[Nick Fantin]

I have found a great alternative to a bubble plot... histogram2 and this covers the requirement of binning by row and column (ie bivariate)