The purpose of error bars in a graph or chart is to measure the uncertainty or variability in the presented data. Error bars are typically used in scientific research, statistical analysis, and presentations to represent the precision and reliability of results visually. These error bars extend vertically from each point on the graph and indicate the range within which the true value is likely to fall. The length of the error bar may be determined by various factors such as sample size, variability of measurements, and statistical tests employed. Larger error bars suggest greater uncertainty, while smaller ones imply higher confidence in the measurements.