Lines of code are very hard to count these days with much of the work done in tools like Visual Studio (if you are a Microsoft shop). But that shouldn't stop a control chart.
A simple control chart can be developed by averaging together the number of total defects from the past 20 projects. The mean number of defects becomes the center line. Now calculate the standard deviation of the average. The upper and lower control limits are normally determined by 3 sigma or 3 standard deviations over and 3 standard deviations under the center line. You should have three parallal lines. Now use the chart to plot the number of defects. It shouldn't matter if they are in the design or in the code -- both are defects. Why argue over which category a bug falls in? Its a bug and needs to be fixed.
Now review the chart. Any points outside of the control lines needs to be questioned -- you need to look for the cause. Another state to watch for is too many points consecutively on one side of the center line. If the defects are random, each event has a probability of 1 in a thousand. Typically 5-7 events on one side consecutively are deemed to be non-random events and also need a probable cause to be determined.