The sparkline tooltip is rendered in-place and can't overflow out of the grid cell.
This is something that we (the Kendo UI team) are looking to address in the next major release.
You can use the following style overrides as a workaround:
.k-sparkline .k-tooltip table {
table-layout: auto;
width: auto;
}
.k-sparkline .k-tooltip table td {
border-left-width: auto;
text-overflow: clip;
}
.k-sparkline .k-tooltip table td,
.k-sparkline .k-tooltip table th {
border-width: 0;
}
.k-grid td {
overflow: visible !important;
}
.k-grid-content {
overflow: visible !important;
}