I have a variable (vitamin D levels) that does not follow normal distribution. I wanted to plot its variation throughout a period of time. I used the median. However I wanted to add error bars in this Excel graph. I used standard deviation but I don't know if it is correct. If it is not, what other measure of dispersion should be used?
You can use a standard deviation error bar (the distribution and the sample do have standard deviations), but if the distribution is asymmetrical then the resulting graph may not communicate the distribution very well.
Consider instead using a dot plot of the whole sample, or a box and whisker plot or, particularly if you have a large number of data points, a simple histogram of the values in your sample.