Adding values month-wise manually to a chart

Hi, 

I have a chart which has values taken from data which is added automatically. Therefore, in the chart I have manipulated in such way that the values come under columns monthly. However, there is a value which requires manual updating of data. In other words, it doesn't come automatically under the data. Therefore, how can I add data manually so that I can put that data in a line form montly withouth affecting my other data. It would be really helpful. Thank you.