In row 1, put in the following headers, then fill in going down. So in the dates, fill in the first few dates, in order, then highlight those, "grab" the lower right corner and drag down to fill. Excel auto fills. Same with the days of the week, do the first 3 or 4 manually, highlight them, then grab the bottom right of the highlighted area and drag to fill. Start your dates in row 2. Start the formula in Column F in row 3.
Column A: Date
Column B: Day of week
Column C: Work day
Column D: Home brought lunches
Column E: Cafeteria / out to lunch lunches
Column F: =sum(Column D) / Sum (Column D and E). In row 12, assuming you've started your date range at row 2, the formula will read exactly: = sum(D$2:D12)/sum(D$2:E12). Note that the "$" in the formula, which you have to add in manually, will insure the noted value doesn't change when you copy this formula down into lower rows (that is, when you copy this to row 13, the 2 will stay 2, instead of changing to 3, which is what would happen without the dollar sign).
For days you bring lunch, put a number 1 in the home column. For days you buy / go out to lunch, put a number 1 in the cafeteria / out to lunch column.
The first sum in the formula in column F will total up the number of home lunches. The 2nd sum will total up all the lunches - both home and cafeteria. Dividing home lunches by total is the percent from home. Use a graph for a visual representation - I'd suggest a scatter plot. Use the dates as the X value and percentage as the Y value.