Overlaid Contour Plot - Overview
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Contour plots show how response variables relate to two continuous variables while holding the rest of the variables in a model at certain settings. In a contour plot, the fitted response model is viewed as a two-dimensional surface where all points that have the same fitted value are connected to produce contour lines of constants. Contour plots are useful for establishing operating conditions that produce desirable response values.

Applications that involve multiple responses present a different challenge than single response experiments. Since each response is important in determining the quality of the product, you need to consider the responses simultaneously. Optimal input variable settings for one response may be far from optimal for another response. Overlaid contour plots allow you to visually identify an area of compromise among the various responses.  

The white regions on the plots are the feasible regions, or the areas that satisfy the criteria for all responses. If you set your factors/components at any of the levels shown in the white area, the responses should fall within the specified ranges.