Determine the sample size or period during which measurements will be made. This should be enough to ensure that the final chart will have bars of varying height, sufficient to compare them. A test of this is that increasing the number of measurements should not change the order or relative height of the bars (unless the process is unstable).
If the measurement unit is a simple frequency, aim for a total of 50 or more measurements.
In any repeat measurement, keep constant any variables which might distort the figures. This includes the sample size or measurement period, along with anything else which might affect the result, such as people, materials, etc. If this cannot be done, then recognize it and take it into account when interpreting the final chart.