The grid is where any initial data from an Open or Import will be visible.
If you tapped "New" then an empty grid will be displayed.
Tapping "Chart" will update the chart display using the grid's data.
Data can be added or amended by simply tapping the grid cell you want to edit and the onscreen keyboard will become visible.
The currently selected cell will be coloured green. Column and row headers can also be edited.
You can restrict which columns are displayed by the chart. This is achieved by selecting columns.
A column can be selected by tapping the highlighted column letter to the left of each column header cell. Tap again to unselect the column.
More than one column can be selected (although some chart types, for instance pie charts will only use the first selected column - or column A if there are no selections)
A "long" tap on a column letter (hold the tap down for a second) will display the Column dialog. This allows you to select/unselect the column, clear the contents of all data cells in that column, or move the whole column one column left or right.