Many pediatricians think that giving a young child pediasure or anything similar just teaches them bad eating habits. It's somewhat acceptable if the child has been sick and unable to get nutrition from food but too often it becomes a substitute for eating healthy food and it's counterproductive.
The weight chart is just an average of what other children weigh. With the tremendous rise in overweight children, why do we worry when our toddler is underweight? I often wonder who they are comparing my children to when they keep coming up underweight at their well checks until I see extremely overweight children at the supermarket, (funny, I never see those overweight children at the park, hmmm, I wonder why?)
Right now your daughter is learning her eating habits. It's up to you to decide what eating habits you are going to teach her. Most toddlers are picky eaters but if you keep offering her a variety of foods, she will eventually become a good eater. If you cater to her food choices, you will be doing that for the rest of your life. Keep offering her a variety of healthy foods often, five times a day if you have to. When she gets hungry, she will eat. If she fills up on milk, or "nutritional shakes" like pediasure, she will never develop an appetite for food. She's only 15 months old so she is still learning how to get her nutrition from food since she probably received most of her nourishment from formula or breastmilk until 12 months old. It can take a long time to make that transition. Just be patient. Don't give up.
Since she doesn't like meat (my four year old still isn't a fan but will eat it now) try giving her greek yogurt which has a lot of protein. You can add cut up fresh fruit or maple syrup or honey for flavor. Be sure to give her a grain with it to make it a complete protein since yogurt alone isn't enough.
Be patient and good luck.