English Tech/Engineering Electronics / Elect Eng Biofeedback machine

In my current translation I am repeatedly needing to describe the output from biofeedback machines - displays of up and down lines on a graph, the lines representing temperature, ECG, skin conductance etc. The question is, is a screen or a printed page of these wavy lines a chart or a diagram? My instinct is to call it a chart (thinking of those temperature charts at the bottom of every hospital patient's bed) - but I suspect the client will press for diagram, as the much-used German word is Diagramm. We need to be able to talk of "creating a chart/diagram" by getting particular information from the computer, and "interpreting/evaluating the chart/diagram".

Should I press for "chart", or is "diagram" perfectly acceptable?