I'm getting a lot of these errors in the browser inspector:
Unexpected value -Infinity% parsing offset attribute. mini-graph-card.js:1:29437
Unexpected value NaN% parsing offset attribute. mini-graph-card.js:1:29437
Graphs seem to be displaying ok.
Example config:
color_thresholds:
- color: '#039BE5'
value: 0
- color: '#0da035'
value: 235
- color: '#e0b400'
value: 250
- color: '#e45e65'
value: 253
color_thresholds_transition: hard
entities:
- sensor.lounge_av_voltage
group: false
hour24: true
line_width: 4
lower_bound: 220
points_per_hour: 1
show:
extrema: true
fill: fade
labels: false
style: |
ha-card {
border: solid 2px var(--primary-color);
}
type: 'custom:mini-graph-card'
upper_bound: 260
Just having a guess here, but the only use of offset followed by a "%" that I could see is line 458 of main.js:
<stop stop-color=${stop.color} offset=${`${stop.offset}%`} />
And offset comes from line 119 of graph.js:
offset: (this._max - stop.value) * (100 / scale),
So if scale was zero, you'd get Infinity. And scale comes from line 106:
const scale = this._max - this._min;
So if the graph had a matching min and max, you would probably get this error.

I've pushed a fix to the dev branch. Can you try it out and see if it fixes the errors? You would need to follow the Development instructions at the bottom of the page to build a copy of the dev branch.
Should be fixed in the latest release, thanks @michaelblight