Your glucose graph is pre-set to have the "green" (green=good) range be 70-140 mg/dL. We want to keep glucose spikes below this 140 threshold as much as possible, but we also want to avoid major shifts in glucose even if it is staying in the green the whole time.
With that being said, you can adjust your glucose thresholds under Settings -> Health Settings.
If you are really looking to take your health to the next level and optimize, you can change your upper threshold to 120. If you are spiking often and want to start with a smaller goal to first master, you can adjust the upper threshold to 160.
Do you often go below 70? Many non-diabetics may actually have glucose readings below 70, especially when fasting. As long as you are not having symptoms of hypoglycemia, such as dizziness, nausea, or sweating, then this is perfectly okay. You can adjust your low threshold if this happens to you often.