Trends regarding data can be shown on graphs with numbers.
The value of stocks and bonds (their prices) can be graphed over time. Those prices are the data that are then put into a graph. The prices can be shown to go, down, up or be somewhat flat.
The same idea can be used (if you have data) for just about any human endeavor.
For example, you can show traffic patterns on roads to coastal areas, and from that create graphs that will give you the "trends" of beach going in summer.
Definition of trend:
pattern of gradual change in a condition, output, or process, or an average or general tendency of a series of data points to move in a certain direction over time, represented by a line or curve on a graph.
Definition of trend
That said, every time someone says "trend" they do not mean they are being scientific or that they have a graph to back up their statement. They generally mean they believe some kind of behavior is more frequent, less frequent, or the same than at some earlier point in time.