How much does donating blood affect the amount of oxygen in your blood stream at high altitude?
The oxygen-carrying capacity of the blood is:


  amount of haemoglobin * % saturation


The average blood donation is approximately 10% of total blood volume. . Since the donation will have no effect on the % saturation, the oxygen-carrying capacity at each altitude will go down by 10%.

By inspection of the graph in the question, the % saturation goes down by 10% at 10,000 feet. So I conclude that donating 10% of blood volume is equivalent to ascending to 10,000 feet in terms of the effect upon the blood's oxygen-carring capacity.

Response to OP comments:

1) Anyone living at 5,000 feet will have adapted to that altitude in two ways. First of all the erythrocyte levels of 2,3 bisphosphoglycerate will be elevated to maintain unloading of oxygen from haemoglobin in the tissues. Secondly, the number of red cells per mL of blood will have increased, to restore oxygen delivery. Incidentally this increases blood viscosity which is deleterious in the long term. Tibetans living in the Himalaya have adapted genetically to prevent this effect on their blood viscosity.

2) The effect of air pressure is already included in the graph, since it shows the actual degree of saturation of the blood.