How do I connect missing points in a Google Sheets Line Chart with a straight line?

When you want to check it using Google Apps Script, you can also use the following script.

function myFunction() {
  const sheet = SpreadsheetApp.getActiveSheet();
  const chart = sheet.getCharts()[0];
  const c = chart.modify().setOption("interpolateNulls", "true").build();
  sheet.updateChart(c);
}

Reference:

• Configuration Options