What it shows
Each calendar year's monthly cost of quality on the same January to December axis: the latest year in the accent colour, up to two earlier years muted.
Why it matters in manufacturing
Many plants have seasons: summer shutdowns, product launches, year-end pushes. Comparing a month with the same month a year earlier separates a real improvement from a seasonal dip.
How to read it
Look at where the bright line runs against the muted ones. Below them month after month is real progress; a dip that the earlier years show in the same month is probably the season.
What good looks like
The current year's line runs below the earlier years in most months, including the season peaks, not only in a single month.