Place a trendline above the bar chart?

Hi, I noticed some bar charts that showed a trendline(I'm guessing it's a trendline) that shows the overall trend above the bars.

eg.

Line above and is a straight line or curve, which does NOT fluctuate with the data

Bar chart below and it fluctuates: 10 30 15 20 32 29

My apologies for the poor visual example, but could someone understand me? Appreciate any help, thanks.