Thanks to both of you for your replies, though neither of them quite
hit the spot.

I can't use a DataGrid, even with TemplateColumns, because I don't know
how many properties there will be down the left-hand side (it's
dependent on product type). Equally, my columns all need the same
template (apart from the header column) -- it's the rows that are the
formatting problem. If you look again at the diagram I drew, you'll
see that (apart from the header column) every column represents one
product, with consistent formatting across the columns; the problem is
that the first cell needs to be very different to the others. And the
second row is one cell, rowspanned across the full row.

Inverting the DataTable might well be of some use to me (and thank you
very much for sharing the code), but rearranging the data wasn't the
intrinsically difficult part; displaying it was.

In the end, one of my colleagues came up with the idea that seems to
have worked -- to use a Table control and mess with the cells
programatically.

I've simplified it, removed lots of the error-handling and complicated
details that weren't relevant to my query and so on, but this is
roughly what I ended up writing (in C#) yesterday:

string[] productIDs = Request.QueryString["p"].Split(',');

ProductCatalog catalog = Microsoft.CommerceServer.Runtime.
CommerceContext.Current.CatalogSystem.GetCatalog(...);

...

#region TableRow row0
TableRow row0 = new TableRow();
CompareTable.Rows.Add(row0);
row0.Cells.Add(new TableCell()); // Empty leftmost cell
#endregion

#region TableRow specsRow
TableRow specsRow = new TableRow();
TableCell specsCell = new TableCell();
Label specsLabel = new Label();
specsLabel.Text = "Specifications";
specsCell.Controls.Add(specsLabel);
specsCell.ColumnSpan = productIDs.Length + 1;
specsRow.Cells.Add(specsCell);
CompareTable.Rows.Add(specsRow);
#endregion

#region TableRow rowN
TableRow rowN = new TableRow();
CompareTable.Rows.Add(rowN);
rowN.Cells.Add(new TableCell());
#endregion

#region Loop through all products...
bool rowHeadersSet = false;
foreach (string productID in productIDs)
{
Trace.Write("Compare", "Inspecting product " + productID
+ "...");

DataRow properties = catalog.GetProduct(productID
).GetProductProperties().Tables[0].Rows[0];
string type = properties["DefinitionName"].ToString();

if (_productType == null)
_productType = type;
else if (_productType != type)
throw new CatalogException(
"Cannot compare products of different types");

string productLink = "~/Product.aspx?CS_ProductID=" + ...;

#region TableCell cell0
TableCell cell0 = new TableCell();

#region HyperLink img
HyperLink img = new HyperLink();
img.NavigateUrl = productLink;
img.ToolTip = properties["DisplayName"].ToString();
img.ImageUrl = ...;
img.CssClass = "compareImage";
cell0.Controls.Add(LiteralOpenPara);
cell0.Controls.Add(img);
cell0.Controls.Add(LiteralClosePara);
#endregion

#region HyperLink name
HyperLink name = new HyperLink();
name.NavigateUrl = productLink;
name.Text = properties["DisplayName"].ToString();
name.CssClass = "compareProductName";
cell0.Controls.Add(LiteralOpenPara);
cell0.Controls.Add(name);
cell0.Controls.Add(LiteralClosePara);
#endregion

#region Label price
Label price = new Label();
price.Text = GenericUtilities.FormatPrice(
(decimal) properties["cy_list_price"]);
price.CssClass = "comparePrice";
cell0.Controls.Add(price);
#endregion

#region Label desc
Label desc = new Label();
desc.Text = properties["Product Intro"].ToString();
desc.CssClass = "compareDesc";
cell0.Controls.Add(desc);
#endregion

#region HyperLink removeLink
HyperLink removeLink = new HyperLink();
removeLink.NavigateUrl = GetRemoveLink(productID);
removeLink.Text = "Remove";
removeLink.CssClass = "compareRemoveLink";
cell0.Controls.Add(LiteralOpenPara);
cell0.Controls.Add(removeLink);
cell0.Controls.Add(LiteralClosePara);
#endregion

row0.Cells.Add(cell0);
#endregion

#region Other cells...
#region string[] keyNames
string[] keyNames = AppSettings.GetCommonSettings(
"ProductSpecs." + _productType).Split(';');

if (!rowHeadersSet)
{
for (int i = 0; i < keyNames.Length; i++)
{
TableRow r = new TableRow();
TableHeaderCell c = new TableHeaderCell();
c.Text = keyNames[i];
c.Attributes["scope"] = "row";
r.Cells.Add(c);
CompareTable.Rows.Add(r);
}

rowHeadersSet = true;
}
#endregion

for (int i = 0; i < keyNames.Length; i++)
{
TableCell cell = new TableCell();

if (...)
{
...
}
else
cell.Text = GenericUtilities.GetNullableString(
properties[keyNames[i]]);

Trace.Write("Compare", "Now adding cell " + (i+2)
+ " of " + CompareTable.Rows.Count);

// Now add the cell, relying on the for loop being
// synchronised with the one inside the if
// (!rowHeadersSet) statement in the code region
// "string[] keyNames".
CompareTable.Rows[i + 2].Cells.Add(cell);
}
#endregion
}
#endregion