The graph above shows the rate of carbon dioxide uptake by a plant over time. Since carbon dioxide is a reactant in photosynthesis, the graph shows the rate of photosynthesis. So, the data shows that photosynthesis is occurring.
The reason for the the increasing rate of photosynthesis is that the plant may be adapting to the increased light intensity inside the glass enclosure. Plant may be closing its stomata (openings on the surface of the leaves) less often, which allows more carbon dioxide to enter the leaves.
So, the plant may be producing more chlorophyll, the green pigment that absorbs sunlight and is essential for photosynthesis. The rate of photosynthesis is increasing over time, which means that the plant is photosynthesizing at a faster rate.