How to plot a cell array in Matlab

You can use ismember to find where each label exists in your cell array. The second output will provide the index of the label. You can then use imagesc with a custom colormap to display the result.

% Create a copy of Grid where the empty cells are replaced with ''
tmp = Grid;
tmp = cellfun(@(x)['' x], Grid, 'UniformOutput', false);

% Locate all of the 's' and 'i' cells and assign values of 1 and 2 respectively
[~, labels] = ismember(tmp, {'s', 'i'});

% Display the resulting label matrix
imagesc(labels)

% Use a custom colormap where empty cells are black, 's' are blue and 'i' are red
cmap = [0 0 0; 0 0 1; 1 0 0];
colormap(cmap)

And if we test this with Grid = {'s', []; 'i', []}

If you want actual dots instead, you can do something like this:

colors = {'r', 'b'};
labels = {'s', 'i'};

for k = 1:numel(labels)
    % Find the row/column indices of the matches
    [r, c] = find(cellfun(@(x)isequal(x, labels{k}), Grid));

    % Plot these at points using the specified color        
    plot(c, r, '.', 'Color', colors{k}, 'MarkerSize', 20);
    hold on
end