Yes I think there is a bug regarding this marker config in sencha charts,

Temporarily I found a solution which works fine for me,

I have replaced below code
Code:
{        "selector": "chart[cls=\"line1\"] series:nth-child(1) marker",
        "style": {
            "type": "circle",
             "size": 2,
            "radius": 2,
            "fill": "#FE1817",
            "stroke-width": 0
        },
        "specificity": [
            0,
            2,
            3
        ]
    },
    {
        "selector": "chart[cls=\"line1\"] series:nth-child(2) marker",
        "style": {
            "type": "plus",
             "size": 2,
            "radius": 2,
            "fill": "#005337",
            "stroke-width": 0
        },
        "specificity": [
            0,
            2,
            3
        ]
    },
    {
        "selector": "chart[cls=\"line1\"] series:nth-child(3) marker",
        "style": {
            "type": "cross",
             "size": 2,
            "radius": 2,
            "fill": "#0C02FB",
            "stroke-width": 0
        },
        "specificity": [
            0,
            2,
            3
        ]
    },
instead of this default code in touch-charts-debug.js and in themeList.js

Code:
{        "selector": "chart[cls=\"line1\"] series:nth-child(1) marker",
        "style": {
            "type": "image",
            "height": "46",
            "width": "46",
            "src": "\"../resources/shared/img/iphone.png\""
        },
        "specificity": [
            0,
            2,
            3
        ]
    },
    {
        "selector": "chart[cls=\"line1\"] series:nth-child(2) marker",
        "style": {
            "type": "image",
            "height": "46",
            "width": "46",
            "src": "\"../resources/shared/img/android.png\""
        },
        "specificity": [
            0,
            2,
            3
        ]
    },
    {
        "selector": "chart[cls=\"line1\"] series:nth-child(3) marker",
        "style": {
            "type": "image",
            "height": "46",
            "width": "46",
            "src": "\"../resources/shared/img/ipad.png\""
        },
        "specificity": [
            0,
            2,
            3
        ]
    },
Now it's working fine for me

Regards,

Nag.