How do I view S3 usage and cost in cost explorer ?

To see cost AND usage in Cost Explorer, you can filter out specific usage types. There are several charges under S3 service, and they will have different pricing units. For example, the actual storage is charged Gb per month; the API calls (like PUT, COPY, GET) are charged for X number of calls; then there may be data transfer (traffic charges) that will also be charged Gb per month. Because of the different pricing units, when you simply look at the total cost for S3, you won't get the usage graph.

I see on the screenshot you are grouping your cost by API operation. To see both cost (top graph) and usage (bottom graph), associated with each individual usage type, you would have to apply additional filters, and look ONLY at the usage types that have the same pricing units. For example... let's say I want to see how much I pay for Standard Storage in S3, the actual Gb/month of storage. I would then apply Usage Type filter, and search for anything that looks like "TimedStorage-ByteHrs"

Alternatively, for some services, there are pre-defined groups of usage types already in the "Usage Type Group" filter. Those are essentially usage types for particular services / features, that are billed with the same pricing unit (like Gb-month for storage or traffic; or Hours for compute usage), grouped for convenience. If you filter with the Usage Type Group, it will output both cost and usage graphs.