When a metric is added to the graph, it is typically added to primary axis of data. This allows you to add multiple metrics to the same graph and have them plotted against each other. You are also able to add the metric data to a secondary axis (upon the right hand side of the graph). This data is separate to the primary Y axis data but the X axis (time) is common.
Please note that you can also customize the secondary axis via the graph configuration (for example, specifying the secondary axis title).