Hi Gene,
I don't know what the colour on B9 is telling you either. My first thought was that it was caused by the text in column A overflowing into column B due to the expanded text used for "Saturday" compared to the rest of the week. But then I noticed the zero in the blackened cell, which would have prevented the text from crossing the cell boundary and entering column B (as can be seen in my example).
As for the chart, here's a reconstructed stacked bar chart using the same layout and data as your example.
the only change I've made from the default settings of the chart as created is to decrease the height of the chart to make it closer to the height of the table. That was done only tomake the chart fit into about the same vertical pace as the table when posted here.
The series labels in the legend above the chart, and the category names along the x axis were taken from the header rows and header column; the values on the Y axis from the range of value sums from columns B and C.

"But I don't know how to specify the y-axis. Nor do I know where to specify the labels."
The upper and lower values of the Y axis are set automatically by Numbers. You can set a Max and Min value and revise the number of steps using the Chart section of the Format Inspector.
What label(s) do you want to place on the y axis?
A possible fix for the black on B9 is to replace that row, following the steps below:
- Click on Friday, then press option-down arrow to insert a new row between Friday and Saturday.
- Reselect the Friday cell.
- Use the yellow fill handle on the bottom of the Friday cell to fill the weekday into the next row. Keep the same font style as used for Friday.
- Re-enter (from the keyboard, not copy/paste) the Saturday values for columns B and C into the new Saturday row.
- Click on any cell in the bottom row, then go (menu) Table>Delete Row.
Regards,
Barry