What is the purpose of a scatter plot?

A scatter plot's primary purpose is to visualize the relationship, or correlation, between two numerical variables. Each point on the plot represents an observation, with its position determined by the values of the two variables. This allows you to quickly identify patterns like positive correlation (as one variable increases, the other tends to increase), negative correlation (as one increases, the other tends to decrease), or no correlation. "Observe trends over time" is best suited for line charts. "Track performance against a target" is typically done with bullet charts or gauges. "Compare multiple parts of a whole" is the domain of pie charts or stacked bar charts.