plots: expand legend character limit

[dberenbaum]

We need to address this in the template:

                            "encoding": {
                                "color": {
                                    "type": "nominal",
                                    "field": "rev",
                                    "legend": {
                                        "labelLimit": 300
                                    }
                                }

Originally posted by @daavoo in iterative/dvc#7477 (comment)