I need help creating a theoretical supply and demand graph for this research question. Is there any other way to visually represent my theory that religion and income are related and that as the percentage of those who say religion is important in their lives increases the annual median income decreases?
There is no way that the relationship that you hypothesize between religion and income can be represented using a supply and demand graph. A supply and demand graph has to have price as its vertical axis and quantity as its horizontal axis. You cannot plot the extent to which religion is important in someone’s life on these axes. You could explain someone’s income using the supply and demand for the profession they work in, but there would be no way to include religiosity on that graph.
What I would suggest is a bar graph. I would put a measure of religiosity on the horizontal axis. I assume that this will be in ordinal form. You will have some sort of measure in which you ask people how important religion is in their lives and they reply on a scale (for example) of 1 to 5. In such a case, you would have five bars on your graph. The vertical axis would be annual income. The mean annual income (or perhaps the median) for each group would determine the height of your bar. In other words, you would take all the people who said that the importance of religion in their life was 1 out of 5, find their mean or median income, and have a bar that went up to that number on the vertical axis. Such a display would easily show a viewer the respective levels of income for people with various degrees of religiosity.