You can use url to a SVG image there as well. However since most SVG images do not define their width/height, you will probably need to add a CSS class as well that specifies the dimensions.
Example: http://jsfiddle.net/z4qk51o7/1/
.toolbar-svg-img > p {
background-size: 20px 35px;
}
var t = new TimeChart({
container: document.getElementById("demo"),
data:
{
units: ["h"],
url: "https://zoomcharts.com/dvsl/data/time-chart/temperature-kuldiga-h.json"
},
toolbar: {
/** This overrides the default buttons. */
items: [
{
item: "back",
align: "left",
image: "path/to/back.svg",
cssClass: "toolbar-svg-img"
},
{ item: "zoomOut", align: "left" },
{ item: "logScale", align: "right" },
{ item: "displayPeriod", align: "right" },
{ item: "displayUnit", align: "right" },
{ item: "export", align: "right" },
]
}
});