In many markets, there is a strong relationship between brand equity and market share. The relationship is strongest in markets where it's easy for customers to switch form one brand to another and weaker in markets that are dominated by brands able to distribute their products more widely than competitors.
A cross plot like the one shown will tell you how strong the relationship is and indicate how much brand equity would need to improve to match a desired increase in market share. Notice that the arrows are not parallel to the line of best fit. This is because a brand that struggles to convert its equity into market share is likely to benefit less from increases in brand equity (i.e. there are other barriers to sales that need to be overcome). Conversely, a brand that already converts well is likely to enjoy a big reward for enhancing its equity.