Data frame with inconsistent time recorded can give misleading result when we use it for plotting line chart. The chart will automatically draw the line across the time period that was skipped in the table, and where the data is missing, to connect together the next recorded part that is far apart. The technic we use to fix this problem is to created data frame with consistent recorded time period from the beginning to the end for every user, then merge this data frame with our original data frame. So we get the newly created data frame with consistent time recorded and "NA" value for the time that data are absent.