Graphical analysis doesn’t improve accuracy of data. It just lets you better visualize what’s happening in your data so that you can understand what physical processes it represents.
If you plot you data on a graph, it does change the values or make them more accurate. But you might see a trend that wasn’t apparent when you were just looking at them as lists of numbers.
often data will give you numbers that are approximatley the same (if you actually have a relationship) rather than giving you the exact same values everytime. this is due in part to experimental errors and fluctuations in conditions. when you plot the numbers on a graph, you can take a statiscal average of the data values that will be an approximated value within some statisical level of uncertainty (depending on the circumstances and the accuracy of the data.)