The metric explorer is available at all levels of the hierarchy via the right click menu. Using that you can browse metrics for all entities below that level of the hierarchy. For example, to look at host metrics, open on a cluster. To look at VM metrics, open on a host or a cluster. You can select the type of entity you want to view metrics for using the drop down on the left hand side. For example, if you open the metric explorer at the data center level, you will be able to select cluster, host and virtual machines. The cog icon lets you view different metrics and measures of that metric as a time series. For example, you can select daily 95th or daily peak time series. You can also look at unaggregated 5 minute data.
Should you wish to show two graphs on a single chart, use the 'compare metrics' toggle switch. Select two metrics. One will be highlighted blue and another orange and will be presented in a single chart using the same color scheme.