You should know the accuracy of the measurement before you plot the results on graph paper, using some sort of error analysis. Often "error bars" are used on a graph to indicate each data point's accuracy. See
http://peltiertech.com/Excel/ChartsHowTo/ErrorBars.html
for an example
How to perform an error analysis is a separate topic. Repeating measurements to determine reproducibility is one ingredient, and so is the precision of measurement devices.
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how to determine the accuracy of an estimate of the size of a dna fragment using a standard curve on semilog graph paper
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