The X-curve shows the relationship between taking care of your responsibility while building your wealth. The theory states that over time, a person's responsibility decreases and their wealth increases.
The Wealth Curve- when you're younger, you still at the early stages of building wealth. Normally, you don't have money. Then as time passes, you start to save and invest which increases your wealth- this can be seen in how your wealth curve is rising. The wealth curve is your investment curve. Hopefully, when you get older, you will have enough money set a side and invested so that you will still earn even when you need to stop working.
The Responsibility Curve- when you younger and starting a new family, the level of responsibility is high. You and your spouse have big job roles to fulfill such as becoming parents and having babies, paying bills, among others. These are obligations that both of you must fulfill whether you live or die. In the early stages of building a family, the need for insurance protection is quite high as well. But as your children grow grow up and your mortgage matures, you reduce your debt. The level of your responsibility then decreases.