Create a Datastudio report
Click on compose new query and then copy and paste the following query into the BigQuery Query editor.

SELECT
  date, SUM(cumulative_confirmed) AS country_cases,
  SUM(cumulative_deceased) AS country_deaths
FROM
  `bigquery-public-data.covid19_open_data.covid19_open_data`
WHERE
  date BETWEEN '2020-03-15'
  AND '2020-04-30'
  AND country_name='United States of America'
GROUP BY date


Click on EXPLORE DATA > Explore with Data Studio.

Authorize Data Studio to access BigQuery.


If may fail to create a report for the first-time login of Data Studio.
Click + Blank Report and accept the Terms of Service.
Then Go back to the BigQuery page and click Explore with Data Studio again.
In the new Data Studio report, select Add a chart > Time-series Chart.

Add country_cases and country_deaths to the Metric field.

Click on Save to commit the change.