Thanks! For the grid, I started with a base value. Every physics tick, which is 60 times per second, a coin is flipped with probability p to be a downward graph or probability 1-p for an upwards graph. Then, a random number is picked from 0% to x% to increase or decrease the previous value.
This enabled me to create the improvement system. If you choose to have a 6% higher chance for the graph to go down, the p of the coin flip is increased to match the graph going down 6% times more. If upward trends are 8% heavier, the random number from 0% to x% gets an 8% higher upper bound for upwards trends.