Styling labels’ background:
(answering this issue)
type: custom:mini-graph-card
entities:
- sensor.xiaomi_cg_1_temperature
show:
labels: true
card_mod:
style: |
ha-card .graph .graph__container .graph__labels span {
background-color: green;
}

Set a color to transparent to make a background less visible:
type: custom:mini-graph-card
entities:
- sensor.xiaomi_cg_1_temperature
show:
labels: true
card_mod:
style: |
ha-card .graph .graph__container .graph__labels span {
background-color: transparent;
}

or even hide it totally:
type: custom:mini-graph-card
entities:
- sensor.xiaomi_cg_1_temperature
show:
labels: true
card_mod:
style: |
ha-card .graph .graph__container .graph__labels span {
background-color: transparent;
box-shadow: none;
}
