Where you have pairs of values and you want to quickly see if there is any correlation (i.e. pattern or trend) you can plot them on a basic x-y graph. For example:
In this case, there doesn’t appear to be any correlation between the values – knowing either x or y gives no clue to what the other might be. We can conclude there is no relationship between the two parameters (based on our limited data set, so far). Whether that’s a useful conclusion depends on what we’re measuring. On the other hand:
There certainly seems to be a trend here – as x increases, so does y (y being around double x). Here, we can conclude there is a relationship – perhaps the x-axis is the sequence number of the reading and is showing us a drift in y. Again, depending on what we’re measuring will dictate whether this is useful to us.
Spreadsheets make it relatively easy for us to sort our data in various ways to look for a whole range of relationships – this is just the starting point.