When you create a "Line" chart in your spreadsheets and one or more value is missing in a series, the line will be broken. To avoid such a situation, go to the menu "Tools/Options/Chart" and check "Interpolated".

By default, hidden rows or columns of data are not shown within a Microsoft Excel chart. It is an interesting feature allowing you to hide rows or columns or to filter the data and show a partial chart in your spreadsheets. If you want all data to be shown, go to the menu "Tools/Options/Chart" and uncheck "Plot visible cells only".