First, I think you need to find a new ped. Yours is obviously undereducated about breastfeeding and probably doesn't like that you are still nursing, which is why he's recommending unnecessary supplementation. Second, your daughter's weight is fine. Weight loss at this age could have nothing to do with diet. Has she been more active lately? Since her prior appointment, perhaps she started crawling, or moving herself around in some way that would cause her to drop some weight. It is such an insignificant drop at a time when many babies are really becoming more active that I think MOST pediatricians would not even think twice about it. If she is meeting all her developmental milestones and wetting enough diapers per day, she is PERFECTLY FINE.
Breastmilk has more calories per ounce than just about anything else that you could give your daughter. If you continue to nurse on demand, before offering solids, she will grow and develop perfectly. Then, when you would already offer solids (if you guys are doing that yet), offer fattier solids, like avocado. That'll just boost the amount of calories in a way that won't affect all the calories she's getting from breastmilk. (Too many solids = less breastfeeding = fewer calories.) At around 9 months to a year, you can start introducing dairy products that aren't straight cow's milk. Whole fat yogurt, cheese, butter, etc. These are also good ways to help her put on a little weight if you're concerned. Just make sure to avoid peanut butter, as I'm sure you don't want to encourage your daughter to have a peanut allergy when she gets older.
In short, you absolutely do not need to supplement with something that is not nearly as good as what you're already giving her.