There are two things you are confusing here:
1.- The willingness to pay for something is completely independent of what the price in the store is. We think of the willingness to pay as something that buyers have regardless of prices. For example, maybe the price of sodas is only \$3 but my willingness to pay is \$10 on really hot days, while more like \$1 on a winter day. Clearly, I will not buy in the winter and definitely buy in the summer. How many will I buy? Well, we don't really have enough information. Maybe the second unit of soda on a summer day is only valuable \$2 to me (in which case I won't buy it), or maybe my willingness to pay is now \$5 (in which case I will buy it). What about the third can of soda in this scenario, we clearly need more information again, etc.
Notice that if the price of soda goes up to \$6, that will have no impact whatsoever on my willingness to pay. It will only have an impact on my decision of whether to buy or not, but not on my intrinsic willingness to pay.
2.- In the demand graph, we always plot price per unit, as explained by @Adam Bailey, so in your example, you have a single point (i.e. when the price is \$1 dollar), so you cannot really say if it's going up or down. Further, you are making something funky with quantities, because what we mean by quantity in the demand graph is "the maximum amount of sodas that will be bought at that price (per soda)". So if the quantity is 100 and the price is \$1.5 the total dollar amount will be \$150, but the price is still \$1.5.
In this last case, we say that "at a price of \$1.5, 100 units are demanded", and conversely you can say that "preferences of consumers are such that exactly 100 units are associated with a willingness to pay of at least \$1.5" (no-one uses the last one, but it may help you connect all the concepts).